Well whatever it was, I now switched "back" to Linux Mint Cinnamon 21.2 and have no issues at all. Even the fractional scaling is working way better than on Kubuntu.

Don't really have an Idea, but to add to your problem...

I have a 4K TV and 2 1080p Monitors and switching beneath them. Using NVIDIA graphics card with up to date drivers. All I can say, I've tried using KDE multiple times now, but always ended up getting weird Bugs, Micro stutter, etc on my 4K TV with KDE. So I always reverted Back to Cinnamon, GNOME, etc, because there it is working fine.