I’ve just tried this out myself, also Debian 12 Stable, I am using Proton Experimental. Before that I’ve also used standalone version through PortProton but I don’t remember doing anything specific for it to run on either RX560 and (nowadays) RX6750 GRE. I doubt that was helpful anyhow, my linux experience is very limited.
UPD: Actually I dug up something in CID:
That supposedly began to happen at 06.06.23 after some Steam update.
↑ this thing is apparently fixed nowadays, by the way. I’ve just successfully switched to full screen and it worked.