I haven’t done the upgrade yet due to all the issues I keep hearing about but an easy first fix I always try is to disable fullscreen optimizations for a game .exe and override the scaling
If 24H2 changed anything with fullscreen apps this may or may not help. I’ve had to change this in the past to get VRR to work when that was still new-ish so it’s not nonsense but whether or not it’ll help is hard to say.
Also see if Game Bar is running in task manager. We can’t turn it off the game bar overlay in settings anymore like in Win 10 so if that was updated and broke something you’ll have to end task every time.