Touch Bar has blank/void button after Catalina update
It could be several things.


Runtime corruption - one program (or several) gets things off course or the macOS code has bugs that will get fixed in an update
Data corruption if this persists across log out / log in and restarts.
Good code and faulty hardware - less likely but could be the cause if it’s intermittent and never able to resolve with updates over weeks or months.


I would triage this by setting up a brand new user account on the Mac, logging out from the one where the bar settings are suspect and then into the newly created empty account.

Should the bar be missing the default icons, you can conclude that the operating system and the bar software need to be reloaded / reset.

If the bar behaves, then it's going to be the settings in your user account that need resetting or fixing.

So to answer the main question - it's a corruption in the settings or the controller and the above steps will help you narrow down which path to remediating it you choose.