If you’re getting content not updating when launching (certain) games fullscreen, but can sometimes bring up the start menu (which makes the game display again), you might be enjoying this particular bug: [BUG] [PSA] : GPU bug causes crashes on KDE Plasma.
For some folks the KWIN_DRM_NO_DIRECT_SCANOUT workaround seems to be fine, for me it doesn’t help at all, and when I play games that struggle like this I’m switching to swaywm for now (or you could use Gnome) until the bug is resolved.