Edit: Solution was just switching 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!

First things first, have you checked protondb.com for My Summer Car entries with similar specs (Arch, GPU, CPU) as yours?

When this has happened to me in the past i've generally been able to fix the issue by adding the launch option gamemoderun %command%. Gamemode tends to fix about 99% of the games I've had issues with.

Tbf i haven't played my summer car so ymmv.

If it has a borderless windowed option, does it produce the same problems?

That's a really strange one, and I have to wonder how the devs programmed it. Games sometimes don't detect my keyboard layout correctly (I use Colemak), but I can't recall a time where the keyboard wasn't recognized at all.

No, it doesn't have that option.
I'm cutting the dev a lot of slack here. He made the game all by himself.
The fix was as easy as switching to a Proton version closer to the game's release date.

Thanks for the reply.
I did, and found someone saying it works fine on Arch with Proton 9, 3 months ago.
So I tried that, and it worked. I had only tried Experimental and 10 before with no success.

Glad you found a solution. In the future, try ALT-Tab in and out of the game and see if that refocuses the window properly. There is a weird bug in Proton 10 that is not raising window focus with Godot, and I'm guessing other engines. It can be fixed by the dev easily, so inquire and see if that is an option.