SUMMARY
I have a ThinkPad T480 laptop with Intel GPU, connected with DELL monitor through HDMI cable.

STEPS TO REPRODUCE
1. Connect laptop to monitor through HDMI
2. Extend display to external monitor
3. Move some window to external screen
4. Unplug the HDMI cable

OBSERVED RESULT

Window in external screen doesn't go back to laptop screen and display manager still think external monitor is connected.

EXPECTED RESULT

Window in external screen should go back to laptop screen and external monitor should be removed from display manager.

SOFTWARE/OS VERSIONS
Windows: 
MacOS: 
Linux/KDE Plasma: openSUSE Tumbleweed
(available in About System)
KDE Plasma Version: 5.14.5
KDE Frameworks Version: 5.54.0
Qt Version: 5.12.0

ADDITIONAL INFORMATION