That might be a kde limitation in general. The amount of "fun" I had dealing with two screens on kde is outright endless. Not sure they even test that kind of configuration, 640x480 pixels should be enough for everyone.
Now imagine they're powered by a docking station, you go into suspend, put the laptop out of the docking station, wake it up and - tadaa! This bug dissappeared but still occurs slightly diffrent for other people. Three monitors itself aren't more edgecase than 2. How long are you using this setup?
Besides screenlockers, having 2 screens with diffrent resolutions is way worse in KDE than in GNOME. (On X11)