final cut not showing full screen on playback monitor in tahoe 26

I use a 4K monitor as playback monitor for FCP, all of a sudden, after installing Tahoe 26, the image does not fill the full screen, it does not matter what resolution I put on my monitor, same is happening if I switch to an HD monitor I have connected as 3rd one. NEVER happened before, tried all what I know, I am out of tries..