to be clear - specific to OBS Studio - I have no idea how to resolve
However, I've seen this sort of thing before. Now whether the Operating System is storing the 'Full Screen' dimensions in registry, and/or if App storing in Registry or config file somewhere??? I suspect .. it depends
So, what I've done is simply toggle from Full Screen to Windowed a few times.. that usually fixes the issues. Possibly changing screen resolution while you are at it.. basically forces Operating Sysam and App to acknowledge something different and reset Full Screen dimensions
I'm guessing you had OBS Studio on Portrait orientation monitor, right?
Is OBS Studio the only app that has this issue? or do other apps going full screen (especially if used previously Full Screen on Portrait orientation) do the same (cover Taskbar)?
If you had Win10, I'd suggest trying a move of the Taskbar to the side, then back to bottom (stupid MS following Apple lead in not allowing such in Win11) which would (hopefully) force a reset. I'm not in front of an OBS system at the moment. the other thing I'd go look at is the OBS config file in \AppData (from memory) and look at values in there in case anything sticks out.