Have you also seen this from Known Issues and Solutions in this very same topic?:
Plasma 6.5 will need manual intervention if you are on X11:
On Plasma 6.5 the Wayland session will be the only one installed when the users does not manually specify kwin-x11.
With the recent split of
kwinintokwin-waylandandkwin-x11, users running the oldX11session needs to manually installplasma-x11-sessionafter the system update completed, or they will not be able to login. Currentlypacmanis not able to figure out your personal setup, and it wouldn’t be ok to installplasma-x11-sessionfor every one using Plasma.
If you wish to continue using X11 you must install plasma-x11-session and reboot, to again be able to select the X11 session from SDDM.
Regards.