docs: update known issues in CHANGES.md with YAML Flatten bug (#35678)

* docs: update known issues in CHANGES.md with YAML Flatten bug

* move this to 2.67

Author: liferoad

CHANGES.md

@@ -139,6 +139,7 @@
 ## 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