Related bug:

Blinking of the windows MAY be related to "kwin". It is suggested to try keyboard shortcut combination Alt+Shift+F12 twice when the issue appear. For me this helps.

Someone said:

The good thing is that disabling / enabling composition gets rid of the issue until next time.
switching between OpenGL versions in the compositor list sets things back on track... for awhile.
You stated that "restarting plasmashell" ("--replace"?) fixed it for you but not for me.