diff --git a/CHANGELOG.md b/CHANGELOG.md
index dea1f44b57..a09f933725 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -15,6 +15,7 @@ IMPROVEMENTS:
 BUG FIXES:
 
 * core: Make sure UIInput keeps working after being canceled [GH-21139]
+* lang/funcs: `flatten` fix handling of sets and tuples; return a tuple [GH-21171]
 * states/statefile: properly upgrade dependency syntax [GH-21159]
 
 ## 0.12.0-beta2 (Apr 18, 2019)