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

 undefined

    
options		nvidia_drm	modeset=1
blacklist	nouveau


  

then edit /etc/mkinitcpio.conf to have

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MODULES=(nvidia nvidia_modeset nvidia_uvm nvidia_drm)


  

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

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HOOKS=(... modconf ...)


  

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