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.