I’d imagine it’s some issue with the multiplane overlays that get composited in the desktop window manager. There were a ton of issues that started up a year or two ago because both Microsoft and Nvidia both flipped it on all the way. The problems weren’t just with UE, but with games and apps as well. Don’t know if it still works, but there’s a registry key you can add that disables it. You’ll have to google around for it, just look up “windows disable mpo.” Make sure to restart the PC after you change it. See if the problem still persists.
Oh and make sure you’re on Windows 10 or 11 21H2 and that you’re fully up to date with system updates.
EDIT: Saved you some digging: After updating to NVIDIA Game Ready Driver 461.09 or newer, some desktop apps may flicker or stutter when resizing the window on some PC configurations | NVIDIA
There’s a couple .reg files you can download to turn it on and off. Again, make sure to restart after using one.