This is definitely a bug that presents itself when using an external keyboard (I’m using Logitech). Auto fill always worked fine until this new OS version. I have confirmed that auto fill works when the external keyboard is disabled. I just upgraded to 15.3.1 a few days ago and have encountered so many bugs, including this one, which is probably one of the most problematic that I’ve found so far.
Note: Apple, if you happen to monitor these forums I’d like to suggest that you stop preventing users from rolling back to a previous version if they so choose. If the previous version is still supported and the current version is buggy and unstable, it is incredibly frustrating to be locked in with no ability to revert. I specifically backed up to my pc before upgrading for this very reason, only to find that you force an update after the restore completes. This absolutely discourages people from willingly installing your updates.