You don’t need to investigate a hardware problem, or system configuration. The issue lies inside your user home folder. You can confirm this by creating a new user account and login to it.
KDE has been rapidly changing a lot of multihead related user settings, so deleting the relevant local *rc file will probably reset (and fix) your issue. IIRC kscreenrc and possibly in some “default” plasma related folder. Keep backups of files and folders you remove for testing (or just rename them, instead of deleting).