The Database Upgrade Assistant (DBUA) by default will remove all underscores and events before actually doing an upgrade. This is expected behaviour as we believe that underscores or events were meant to treat misbehaviour of the database just in a specific release only. And in our experience a nice collection of underscores will at least make your upgrades run slower.
Now with Oracle Database 188.8.131.52 and Oracle Database 184.108.40.206 the DBUA has a new parameter which can be used during startup:
$> dbua -keepHiddenParams
Then it will keep the hidden/underscore parameters during and after the upgrade.
PS: Many thanks to Mr. Frank Becker for highlighting this to myself – I wasn’t aware of it – screenshot is courtesy from Mr Becker as well