Many users updating to Plasma 6 have been experiencing graphic issues due to the fact that the update defaults their login session to Wayland.
Could you log out and switch to X11 at the bottom left of the login screen and see if you still have this issue.
Also, you may want to create a new topic for this one with a descriptive title to get the attention of those Plasma user more in the know in how to handle this X11-Wayland “debacle” (exaggerating) that this recent upgrade has brought about.