Keyboard not working in full-screen Steam game on Wayland

Edit: Solution was just to switch to Proton 9

This is a weird one involving many moving parts, so I'm looking for some pointers on where I can even start trouble-shooting:

• When I open My Summer Car (on Steam) in fullscreen mode, the keyboard doesn't work in game.
• When I start it in windowed mode, it works normally.
• When I then select full screen in the window menu, it works partially (some buttons in game don't react).
• Other games I tested have no issue.
• It works fine on Debian, but the issue is present on more bleeding edge distros (tested Arch, Endeavor and Bazzite)
• Flatpak or native Steam makes no difference
• Only tested KDE Plasma with Wayland

My best guess is that the issue is related to Wayland somehow, and that the game doesn't mesh with the newer Plasma version, but I'd love to know if there's a way to troubleshoot or work around this.Thanks in advance!