help xbox gamebar isn't full screen

Same here, a possible workaround is to pin the party so when not in gamebar, it's on the screen but smaller than the full gamebar. Not found a full solution yet.

The Windows Gamebar can only record one Window at a time or one full screen game at a time.

To record 'Many Windows' or the Windows Desktop will require a third party software application.

gamebar only records the main screen,

Hopefully this thread can help you out. It may be a processor problem but i believe you can’t use your other screen
while trying to use the gamebar

I was having the same problem, the problem is that xbox gamebar isn't actually installed.

Download the Xbox app from the store, then login and click your profile-->settings-->general, and it should show you a message at the top saying xbox game bar isn't installed and it will install it for you.