This happens only with kwin_wayland not kwin_x11.

A good application to test this with is shockolate but needs external game data.
https://github.com/Interrupt/systemshock

1. start shockolate
2. start a new game
3. press space to get rid of the keybinding view
4. press f to enable mouselook (activates mouse lock)

Under wayland the following behaviour can be observed:
1. moving the mouse will not change the camera view in game
(works on X11, my guess is the window doesnt receive mouse events anymore)
2. pressing f again to deactivate mouse look doesnt work
(might be an engine bug, should work on X11, need to recheck)

If the shockolate window isnt fullscreen(the default):
3. switching to another non fullscreen window will not make the
cursor usable again depending on where the (invisible) cursor is atm
(if it is within the switched to window it will appear and be usable otherwise it wont)