diff --git a/lucene/CHANGES.txt b/lucene/CHANGES.txt
index 2af2148..99679bb 100644
--- a/lucene/CHANGES.txt
+++ b/lucene/CHANGES.txt
@@ -7,7 +7,9 @@
 
 Bug Fixes
 ---------------------
-(No changes)
+
+* LUCENE-9300: Fix corruption of the new gen field infos when doc values updates are applied on a segment created
+  externally and added to the index with IndexWriter#addIndexes(Directory). (Jim Ferenczi, Adrien Grand)
 
 ======================= Lucene 8.5.0 =======================
 
diff --git a/lucene/core/src/java/org/apache/lucene/index/ReadersAndUpdates.java b/lucene/core/src/java/org/apache/lucene/index/ReadersAndUpdates.java
index 73f7d66..0bb4b5d 100644
--- a/lucene/core/src/java/org/apache/lucene/index/ReadersAndUpdates.java
+++ b/lucene/core/src/java/org/apache/lucene/index/ReadersAndUpdates.java
@@ -551,27 +551,37 @@
       
       try {
         // clone FieldInfos so that we can update their dvGen separately from
-        // the reader's infos and write them to a new fieldInfos_gen file
-        FieldInfos.Builder builder = new FieldInfos.Builder(fieldNumbers);
-        // cannot use builder.add(reader.getFieldInfos()) because it does not
-        // clone FI.attributes as well FI.dvGen
+        // the reader's infos and write them to a new fieldInfos_gen file.
+        int maxFieldNumber = -1;
+        Map<String, FieldInfo> byName = new HashMap<>();
         for (FieldInfo fi : reader.getFieldInfos()) {
-          FieldInfo clone = builder.add(fi);
-          // copy the stuff FieldInfos.Builder doesn't copy
-          for (Entry<String,String> e : fi.attributes().entrySet()) {
-            clone.putAttribute(e.getKey(), e.getValue());
-          }
-          clone.setDocValuesGen(fi.getDocValuesGen());
+          // cannot use builder.add(fi) because it does not preserve
+          // the local field number. Field numbers can be different from
+          // the global ones if the segment was created externally (and added to
+          // this index with IndexWriter#addIndexes(Directory)).
+          byName.put(fi.name, cloneFieldInfo(fi, fi.number));
+          maxFieldNumber = Math.max(fi.number, maxFieldNumber);
         }
 
         // create new fields with the right DV type
+        FieldInfos.Builder builder = new FieldInfos.Builder(fieldNumbers);
         for (List<DocValuesFieldUpdates> updates : pendingDVUpdates.values()) {
           DocValuesFieldUpdates update = updates.get(0);
-          FieldInfo fieldInfo = builder.getOrAdd(update.field);
-          fieldInfo.setDocValuesType(update.type);
+
+          if (byName.containsKey(update.field)) {
+            // the field already exists in this segment
+            FieldInfo fi = byName.get(update.field);
+            fi.setDocValuesType(update.type);
+          } else {
+            // the field is not present in this segment so we clone the global field
+            // (which is guaranteed to exist) and remaps its field number locally.
+            assert fieldNumbers.contains(update.field, update.type);
+            FieldInfo fi = cloneFieldInfo(builder.getOrAdd(update.field), ++maxFieldNumber);
+            fi.setDocValuesType(update.type);
+            byName.put(fi.name, fi);
+          }
         }
-        
-        fieldInfos = builder.finish();
+        fieldInfos = new FieldInfos(byName.values().toArray(new FieldInfo[0]));
         final DocValuesFormat docValuesFormat = codec.docValuesFormat();
         
         handleDVUpdates(fieldInfos, trackingDir, docValuesFormat, reader, newDVFiles, maxDelGen, infoStream);
@@ -652,6 +662,12 @@
     return true;
   }
 
+  private FieldInfo cloneFieldInfo(FieldInfo fi, int fieldNumber) {
+    return new FieldInfo(fi.name, fieldNumber, fi.hasVectors(), fi.omitsNorms(), fi.hasPayloads(),
+        fi.getIndexOptions(), fi.getDocValuesType(), fi.getDocValuesGen(), new HashMap<>(fi.attributes()),
+        fi.getPointDimensionCount(), fi.getPointIndexDimensionCount(), fi.getPointNumBytes(), fi.isSoftDeletesField());
+  }
+
   private SegmentReader createNewReaderWithLatestLiveDocs(SegmentReader reader) throws IOException {
     assert reader != null;
     assert Thread.holdsLock(this) : Thread.currentThread().getName();
diff --git a/lucene/core/src/test/org/apache/lucene/index/TestNumericDocValuesUpdates.java b/lucene/core/src/test/org/apache/lucene/index/TestNumericDocValuesUpdates.java
index 643daa2..9f6683c 100644
--- a/lucene/core/src/test/org/apache/lucene/index/TestNumericDocValuesUpdates.java
+++ b/lucene/core/src/test/org/apache/lucene/index/TestNumericDocValuesUpdates.java
@@ -36,6 +36,7 @@
 import org.apache.lucene.document.Document;
 import org.apache.lucene.document.Field.Store;
 import org.apache.lucene.document.Field;
+import org.apache.lucene.document.LongPoint;
 import org.apache.lucene.document.NumericDocValuesField;
 import org.apache.lucene.document.SortedDocValuesField;
 import org.apache.lucene.document.SortedSetDocValuesField;
@@ -1483,6 +1484,160 @@
     IOUtils.close(dir1, dir2);
   }
 
+  public void testAddNewFieldAfterAddIndexes() throws Exception {
+    Directory dir1 = newDirectory();
+    IndexWriterConfig conf = newIndexWriterConfig(new MockAnalyzer(random())).setMergePolicy(NoMergePolicy.INSTANCE);
+    final int numDocs = atLeast(50);
+    try (IndexWriter writer = new IndexWriter(dir1, conf)) {
+      // create first index
+      for (int i = 0; i < numDocs; i++) {
+        Document doc = new Document();
+        doc.add(new StringField("id", Integer.toString(i), Store.NO));
+        doc.add(new NumericDocValuesField("a1", 0L));
+        doc.add(new NumericDocValuesField("a2", 1L));
+        writer.addDocument(doc);
+      }
+    }
+
+    Directory dir2 = newDirectory();
+    conf = newIndexWriterConfig(new MockAnalyzer(random())).setMergePolicy(NoMergePolicy.INSTANCE);
+    try (IndexWriter writer = new IndexWriter(dir2, conf)) {
+      // create second index
+      for (int i = 0; i < numDocs; i++) {
+        Document doc = new Document();
+        doc.add(new StringField("id", Integer.toString(i), Store.NO));
+        doc.add(new NumericDocValuesField("i1", 0L));
+        doc.add(new NumericDocValuesField("i2", 1L));
+        doc.add(new NumericDocValuesField("i3", 2L));
+        writer.addDocument(doc);
+      }
+    }
+
+    Directory mainDir = newDirectory();
+    conf = newIndexWriterConfig(new MockAnalyzer(random())).setMergePolicy(NoMergePolicy.INSTANCE);
+    try (IndexWriter writer = new IndexWriter(mainDir, conf)) {
+      writer.addIndexes(dir1, dir2);
+
+      List<FieldInfos> originalFieldInfos = new ArrayList<>();
+      try (DirectoryReader reader = DirectoryReader.open(writer)) {
+        for (LeafReaderContext leaf : reader.leaves()) {
+          originalFieldInfos.add(leaf.reader().getFieldInfos());
+        }
+      }
+      assertTrue(originalFieldInfos.size() > 0);
+
+      // update all doc values
+      long value = random().nextInt();
+      NumericDocValuesField[] update = new NumericDocValuesField[numDocs];
+      for (int i = 0; i < numDocs; i++) {
+        Term term = new Term("id", new BytesRef(Integer.toString(i)));
+        writer.updateDocValues(term, new NumericDocValuesField("ndv", value));
+      }
+
+      try (DirectoryReader reader = DirectoryReader.open(writer)) {
+        for (int i = 0; i < reader.leaves().size(); i++) {
+          LeafReader leafReader = reader.leaves().get(i).reader();
+          FieldInfos origFieldInfos = originalFieldInfos.get(i);
+          FieldInfos newFieldInfos = leafReader.getFieldInfos();
+          ensureConsistentFieldInfos(origFieldInfos, newFieldInfos);
+          assertEquals(DocValuesType.NUMERIC, newFieldInfos.fieldInfo("ndv").getDocValuesType());
+          NumericDocValues ndv = leafReader.getNumericDocValues("ndv");
+          for (int docId = 0; docId < leafReader.maxDoc(); docId++) {
+            assertEquals(docId, ndv.nextDoc());
+            assertEquals(ndv.longValue(), value);
+          }
+        }
+      }
+    }
+    IOUtils.close(dir1, dir2, mainDir);
+  }
+
+  public void testUpdatesAfterAddIndexes() throws Exception {
+    Directory dir1 = newDirectory();
+    IndexWriterConfig conf = newIndexWriterConfig(new MockAnalyzer(random())).setMergePolicy(NoMergePolicy.INSTANCE);
+    final int numDocs = atLeast(50);
+    try (IndexWriter writer = new IndexWriter(dir1, conf)) {
+      // create first index
+      for (int i = 0; i < numDocs; i++) {
+        Document doc = new Document();
+        doc.add(new StringField("id", Integer.toString(i), Store.NO));
+        doc.add(new NumericDocValuesField("ndv", 4L));
+        doc.add(new NumericDocValuesField("control", 8L));
+        doc.add(new LongPoint("i1", 4L));
+        writer.addDocument(doc);
+      }
+    }
+
+    Directory dir2 = newDirectory();
+    conf = newIndexWriterConfig(new MockAnalyzer(random())).setMergePolicy(NoMergePolicy.INSTANCE);
+    try (IndexWriter writer = new IndexWriter(dir2, conf)) {
+      // create second index
+      for (int i = numDocs; i < numDocs * 2; i++) {
+        Document doc = new Document();
+        doc.add(new StringField("id", Integer.toString(i), Store.NO));
+        doc.add(new NumericDocValuesField("ndv", 2L));
+        doc.add(new NumericDocValuesField("control", 4L));
+        doc.add(new LongPoint("i2", 16L));
+        doc.add(new LongPoint("i2", 24L));
+        writer.addDocument(doc);
+      }
+    }
+
+    Directory mainDir = newDirectory();
+    conf = newIndexWriterConfig(new MockAnalyzer(random())).setMergePolicy(NoMergePolicy.INSTANCE);
+    try (IndexWriter writer = new IndexWriter(mainDir, conf)) {
+      writer.addIndexes(dir1, dir2);
+
+      List<FieldInfos> originalFieldInfos = new ArrayList<>();
+      try (DirectoryReader reader = DirectoryReader.open(writer)) {
+        for (LeafReaderContext leaf : reader.leaves()) {
+          originalFieldInfos.add(leaf.reader().getFieldInfos());
+        }
+      }
+      assertTrue(originalFieldInfos.size() > 0);
+
+      // update some docs to a random value
+      long value = random().nextInt();
+      Term term = new Term("id", new BytesRef(Integer.toString(random().nextInt(numDocs) * 2)));
+      writer.updateDocValues(term, new NumericDocValuesField("ndv", value),
+          new NumericDocValuesField("control", value*2));
+
+      try (DirectoryReader reader = DirectoryReader.open(writer)) {
+        for (int i = 0; i < reader.leaves().size(); i++) {
+          LeafReader leafReader = reader.leaves().get(i).reader();
+          FieldInfos origFieldInfos = originalFieldInfos.get(i);
+          FieldInfos newFieldInfos = leafReader.getFieldInfos();
+          ensureConsistentFieldInfos(origFieldInfos, newFieldInfos);
+          assertNotNull(newFieldInfos.fieldInfo("ndv"));
+          assertEquals(DocValuesType.NUMERIC, newFieldInfos.fieldInfo("ndv").getDocValuesType());
+          assertEquals(DocValuesType.NUMERIC, newFieldInfos.fieldInfo("control").getDocValuesType());
+          NumericDocValues ndv = leafReader.getNumericDocValues("ndv");
+          NumericDocValues control = leafReader.getNumericDocValues("control");
+          for (int docId = 0; docId < leafReader.maxDoc(); docId++) {
+            assertEquals(docId, ndv.nextDoc());
+            assertEquals(docId, control.nextDoc());
+            assertEquals(ndv.longValue()*2, control.longValue());
+          }
+        }
+        IndexSearcher searcher = new IndexSearcher(reader);
+        assertEquals(numDocs, searcher.count(LongPoint.newExactQuery("i1", 4L)));
+        assertEquals(numDocs, searcher.count(LongPoint.newExactQuery("i2", 16L)));
+        assertEquals(numDocs, searcher.count(LongPoint.newExactQuery("i2", 24L)));
+      }
+    }
+    IOUtils.close(dir1, dir2, mainDir);
+  }
+
+  private void ensureConsistentFieldInfos(FieldInfos old, FieldInfos after) {
+    for (FieldInfo fi : old) {
+      assertNotNull(after.fieldInfo(fi.number));
+      assertNotNull(after.fieldInfo(fi.name));
+      assertEquals(fi.number, after.fieldInfo(fi.name).number);
+      assertTrue(fi.getDocValuesGen() <= after.fieldInfo(fi.name).getDocValuesGen());
+    }
+  }
+
+
   public void testDeleteUnusedUpdatesFiles() throws Exception {
     Directory dir = newDirectory();
     IndexWriterConfig conf = newIndexWriterConfig(new MockAnalyzer(random()));