I have added an option to disable gamescope in a new release:
https://gitlab.com/patlefort/project...ses/2026.04.01
Can you try? If it still doesn't work then you have a cursed system. If it does, note that you have to alt+enter to put the game back into fullscreen if you alt+tab out and back in.
If it doesn't, maybe consider trying a different distro, updating Ubuntu to the latest or try a different desktop environment.