We tested the game only on LTS versions, but from my quick search it appears KDE 6.3 is using Wayland by default, which the game does not support properly for now, but thanks a lot for feedback! It behaves like this because Wayland does not allow mouse access to apps outside of their windows.
If you would like to you could try and switch session to x11, but i can see how it could be problematic. Wayland is still experimental and a lot of programs and especially games work better on x11. There should be an option during login, a gear icon or something similar in bottom corner.