SUMMARY
On Wayland when you connect an external screen to a laptop and, using the quick display configuration widget or popup, first select "Switch to external display" (leftmost option), then to "Switch to internal display" (or vice versa), nothing happens. In order to switch to the external display, a different option (either extend to the left/right or have them both in sync)
As my system is set to German I'm not sure exactly what the options are called in English.

STEPS TO REPRODUCE
1.  Be on Wayland on a laptop with an external display
2. Using the quick display configuration applet or the Meta+P popup, select "Switch to external display"
3. After that, select "Switch to external display" (won't work)
4. Select "Extend to the right"
5. Try "Switch to external display" again (now it works)

OBSERVED RESULT
"Switch to external display" does not work while only the internal display is active, only if both are active (same for "Switch to internal display")

EXPECTED RESULT
The option should do what it says it does

SOFTWARE/OS VERSIONS
Operating System: KDE Neon Unstable
KDE Plasma Version: 5.23.80
KDE Frameworks Version: 5.89.0
Qt Version: 5.15.3
Graphics Platform: Wayland