Start by checking what windowing system you're using as its a fundamental part of problem solving. It's a little confusing how to do this, the top answer in this Stack exchange thread works well.

If you're running the latest KDE then you've almost certainly been moved to Wayland and that will be the source of your problems. Wayland and Nvidia drivers don't work well together, and KDE have defaulted to Wayland in the latest release. I have had very similar issues to you with the move to wayland and have not been able to fix them - they're too fundamental and depend on updates to wayland and/or Nvidia drivers.

I know you don't want a solution but there isn't one at the moment, so you'd be wasting your time. The solution is to log out, then on the log in screen select Plasma (X11) as your session and log in again.

Personally I have had to abandon KDE as I get a different set of problems in X11. I'm on OpenSuSE Tumbleweed so have little choice inrolling back to the previously functioning version of KDE - I'm using Cinnamon instead and contemplating switching to a different Linux distro, probably OpenSuSE Leap in favour of stability over cutting edge.

Meanwhile I have the latest KDE running on another device with AMD GPU without issue.

In terms of when it'll be fixed, there is a change being made to Wayland which will effect how it and the Nvidia drivers interact (something called Explicit sync). It's just been merged into wayland so presumably will appear downstream in the coming next few months in rolling distributions. There have been articles suggesting this is going to fix most problems but personally I think this is a little brave but fingers crossed.