docs: update known issues in CHANGES.md with YAML Flatten bug (#35681)

Amar3tto · web-flow · commit d653c115e309 · 2025-07-25T08:42:25.000+04:00
docs: update known issues in CHANGES.md with YAML Flatten bug (#35678)
diff --git a/CHANGES.md b/CHANGES.md
@@ -109,6 +109,7 @@
 ## Known Issues
 
 * ([#X](https://github.com/apache/beam/issues/X)).
+* ([#35666](https://github.com/apache/beam/issues/35666)). YAML Flatten incorrectly drops fields when input PCollections' schema are different. This issue exists for all versions since 2.52.0.
 
 # [2.66.0] - 2025-07-01
 
