Submit a bug report to Steam.

I'm guessing you forgot to add DRM kernel mode setting. What you need to do is create /etc/modprobe.d/nvidia.conf, and add

options		nvidia_drm	modeset=1
blacklist	nouveau

then edit /etc/mkinitcpio.conf to have

MODULES=(nvidia nvidia_modeset nvidia_uvm nvidia_drm)

and ensure /etc/mkinitcpio.conf also has the modconf hook

HOOKS=(... modconf ...)

then run mkinitcpio -P, restart, and enjoy Wayland.