I suggest disabling kscreen2 service from KDE settings, as it is probably what is causing you trouble in multi monitor setup. The downside is that it doesn’t remember monitor configurations then when kscreen2 is not active. However you can still manage that with other tools or startup scripts to set your monitors as you wish on login (and/or with a proper xorg config I guess). It has been multiple release and they did not fix this multimonitor mess yet as far as I know. Here is what is affecting me maybe you were having similar/related issue.