According to the debian wiki support is not fully guaranteed. Might just be that you ran into any of those issues:
Wayland support is still not finished yet though. Notably, there are issues with the compositor taking down other applications with it when it crashes, changing the global theme sometimes causes a session crash, “Activities” are unimplemented, there are various papercuts with drag-and-drop functionality, and session restoration only works with XWayland windows. A full and more regularly-updated list of Plasma Wayland issues can be found here: https://community.kde.org/Plasma/Wayland_Showstoppers
Unfortunately X11 isn’t without issues as well. In my case Wayland runs smoother and is less error prone than X11.