diff --git a/rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/search.vectors/60_dense_vector_dynamic_mapping.yml b/rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/search.vectors/60_dense_vector_dynamic_mapping.yml
index d2c02fcbff38e..4ef700f807c13 100644
--- a/rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/search.vectors/60_dense_vector_dynamic_mapping.yml
+++ b/rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/search.vectors/60_dense_vector_dynamic_mapping.yml
@@ -3,6 +3,30 @@ setup:
       version: ' - 8.10.99'
       reason: 'Dynamic mapping of floats to dense_vector was added in 8.11'
 
+  # Additional logging for issue: https://github.com/elastic/elasticsearch/issues/100502
+  - do:
+      cluster.put_settings:
+        body: >
+          {
+            "persistent": {
+              "logger.org.elasticsearch.index": "TRACE"
+            }
+          }
+
+---
+teardown:
+  - skip:
+      version: ' - 8.10.99'
+      reason: 'Dynamic mapping of floats to dense_vector was added in 8.11'
+
+  - do:
+      cluster.put_settings:
+          body: >
+            {
+              "persistent": {
+                "logger.org.elasticsearch.index": null
+              }
+            }
 ---
 "Fields with float arrays below the threshold still map as float":
 
diff --git a/server/src/test/java/org/elasticsearch/index/mapper/vectors/DenseVectorFieldMapperTests.java b/server/src/test/java/org/elasticsearch/index/mapper/vectors/DenseVectorFieldMapperTests.java
index 2899dab6ff303..183c0083c7da1 100644
--- a/server/src/test/java/org/elasticsearch/index/mapper/vectors/DenseVectorFieldMapperTests.java
+++ b/server/src/test/java/org/elasticsearch/index/mapper/vectors/DenseVectorFieldMapperTests.java
@@ -20,7 +20,9 @@
 import org.apache.lucene.search.FieldExistsQuery;
 import org.apache.lucene.search.Query;
 import org.apache.lucene.util.BytesRef;
+import org.elasticsearch.common.bytes.BytesReference;
 import org.elasticsearch.common.util.BigArrays;
+import org.elasticsearch.common.xcontent.XContentHelper;
 import org.elasticsearch.index.IndexVersion;
 import org.elasticsearch.index.codec.CodecService;
 import org.elasticsearch.index.codec.PerFieldMapperCodec;
@@ -231,6 +233,26 @@ public void testDims() {
         }
     }
 
+    public void testMergeDims() throws IOException {
+        XContentBuilder mapping = mapping(b -> {
+            b.startObject("field");
+            b.field("type", "dense_vector");
+            b.endObject();
+        });
+        MapperService mapperService = createMapperService(mapping);
+
+        mapping = mapping(b -> {
+            b.startObject("field");
+            b.field("type", "dense_vector").field("dims", 4).field("similarity", "cosine").field("index", true);
+            b.endObject();
+        });
+        merge(mapperService, mapping);
+        assertEquals(
+            XContentHelper.convertToMap(BytesReference.bytes(mapping), false, mapping.contentType()).v2(),
+            XContentHelper.convertToMap(mapperService.documentMapper().mappingSource().uncompressed(), false, mapping.contentType()).v2()
+        );
+    }
+
     public void testDefaults() throws Exception {
         DocumentMapper mapper = createDocumentMapper(fieldMapping(b -> b.field("type", "dense_vector").field("dims", 3)));