After trying to use old/great version (5.600) as long as possible now I was forced to move to this unified folder craziness (v.9.0) after google ditched support for email access with old version…
So… issue is this with this new version. I have 2 accounts with google (both custom domains) and no matter what settings I tried in K-9, only one account working with Push setting (so that notifications of incoming emails arrive instantly/near instantly).
Other e-mail ignores push notification toggle enabled setting completely.
UPDATE: Got it working by simply toggling “Push” toggle OFF and then ON. Very weird.