Found the commit I was thinking of MnemonicAttached: Fix logic when pressing Alt (9f42be5c) · Commits · Frameworks / Kirigami · GitLab
This went in 6 days ago, so it’s possible this fixes it, and that I have the fix but you don’t yet.
I just checked by reverting that commit, and sure enough: it’s broken now.
So the issue is fixed, and the fix will be released as a part of Frameworks 6.2.