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.
Submit a bug report to Steam.