I set up my outlook.com account as Exchange, instead of the default IMAP, for push instead of poll. It broke right after play store updated Gmail a few days ago.
Googling "gmail app exchange security settings are more secure" I notice the problem first occurred in
April with the Android 10 update. Seem like google doesn't bother fixing bugs before subjecting the world to them!
Workaround is to roll back by "uninstalling", which actually reverts to the Gmail version that originally came with your Android version.