I found a similar problem to mine from this post 3 years ago:
https://www.reddit.com/r/kde/comments/6n4vo0/on_resume_from_suspend_i_get_desktop_effects_were/

I am facing the same issues when I leave applications such as Google Chrome open before suspending my KDE session. When I wake it up, KWin says that "desktop effects were restarted due to a graphics reset", and I see black patches all over which only disappear if I wave my mouse over them. However, Chrome's graphics are messed up for the rest of the session (e.g. logo bleeding in to search results, tabs not changing properly to name a few). Similarly, I see black when I open KDE system settings where the left sidebar should be, but this blank does not disappear if I wave my mouse over it.

I'm pretty sure the graphics are messed up throughout, but I was too scared to open anything else.

I know the nvidia proprietary drivers I use have long had problems with Linux DEs and WMs (I experience the same issues on Xfce and XMonad), so I was wondering whether anyone still has the same problems I do and whether they managed to fix them. I also want to know whether nvidia is planning on fixing any of this in the meantime.

I did try changing the compositor settings, but enabling/disabling on startup and changing to XRenderer didn't seem to affect the bug. I also tried reinstalling nvidia and nvidia-utils, but that doesn't seem to work.

One solution that *sort of* works is logging out and then logging back in. Graphics seem to return to normal that way, but I lose the windows from my session due to another problem with KDE in which it doesn't save my last session even if I have that setting activated. Regardless, re-logging in is kind of annoying - even if I do manage to get my last session saved - so I really do hope somebody can help me fix this problem soon. Thanks!

You need to be more precise about your setup: distribution, exact Plasma version?