I have three monitors, and the plamsa task manager seems to mix up which
monitor is which, and say windows are on the wrong screen. I have all three of
my task managers (one per screen) set to only show windows from the same
screen, desktop and activity.
This has been an ocassional issue for a while, but it has just gotten a lot
worse after some recent updates. It used to be that a plasma restart or
reconfig would fix it, but that no longer works. I even tried deleting a panel
and re-adding it to see if that would fix its confusion. It did not.