update limitations.md from limitations.rst

Author: kylehounslow

docs/user/ppl/limitations/limitations.md

@@ -62,4 +62,28 @@ For the following functionalities, the query will be forwarded to the V2 query e
     * ML  
     * Kmeans  
 * `show datasources` and command  
-* Commands with `fetch_size` parameter  
+* Commands with `fetch_size` parameter  
+
+
+## Malformed Field Names in Object Fields
+
+OpenSearch normally rejects field names containing problematic dot patterns (such as `.`, `..`, `.a`, `a.`, or `a..b`). However, when an object field has `enabled: false`, OpenSearch bypasses field name validation and allows storing documents with any field names.
+
+If a document contains malformed field names inside an object field, PPL ignores those malformed field names. Other valid fields in the document are returned normally.
+
+**Example of affected data:**
+
+```json
+{
+    "log": {
+    ".": "value1",
+    ".a": "value2",
+    "a.": "value3",
+    "a..b": "value4"
+    }
+}
+```
+
+When ``log`` is an object field with ``enabled: false``, subfields with malformed names are ignored.
+
+**Recommendation:** Avoid using field names that contain leading dots, trailing dots, consecutive dots, or consist only of dots. This aligns with OpenSearch's default field naming requirements.