Could this be a problem with the handling of hot-plug events? In particular Display-Port connected monitors seem to give away more info about their connection/power state.
One experiment that might be worth a try is disable KScreen2 which seems to take action on hot-plugging of monitors. This can be easily done via the Background Services application which used to be part of Settings, but is now separate, and can be found by using the launcher to search for Background Services.