This may be an issue with Steam, rather than the Xwindow system or Wayland. I am not 100% sure on that.
But, you can read this users experience and their workaround here:
I think they are talking about the Steam Client running natively. I'm using Steam Link which is essentially a remote desktop to the steam client that I have running on another puter.
I hadn't tried it on this computer for a while and I'm pretty sure it was working previously.
Primary Display is hdmi, Secondary display is Display Port.
I just tried unplugging the Display Port, ran Steam Link so was open on my Primary/HDMI and then when I plugged the second display back in, it immediately flicked back over to the second display.
Are there any key commands which move the windows between displays?