Okay, it was a bit of an adventure but this looks like an issue with how the game behaves based on EDID values. An important detail I missed was that OLED Deck is *also* broken, but only on desktop mode. On both LCD Deck...
I compared this between LCD Deck and OLED Deck and was able to reproduce a similar issue on my LCD Deck while my OLED Deck worked fine. I checked SteamOS Stable and SteamOS Main on both, and it didn't seem to make a diff...