Doesn’t seem to be a bug.

here’s an issue on GitHub for GE-Proton about this problem, where they blame it on Valve.

As I understand it, PROTON_ENABLE_WAYLAND is needed for HDR, at least on gnome, which is what I am using.

I think KDE might have somewhat better HDR support right now, but I much prefer gnome and am trying to use Bazzite as a daily driver besides just games.