SUMMARY Using the Win+P switch, it's not possible to select "laptop screen" if "external monitor" is currently set. If one wants to switch to "laptop screen" they have to switch to something else (like "clone") first and then switch to "laptop screen". This was tested on Wayland.
Can confirm on neon unstable. I have mentioned this bug recently in bug 419741 comment 3.
Can also confirm.
Can no longer confirm on Plasma 6; seems to be fixed now!