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.
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.