Yeah, sure. 4k TV, deck and game both output 1080p.
I was able to fix it by changing the resolution for the game in the steam settings for the game, under "properties" on the game's page.
(i had to add this to the steam launcher to play it on my TV)
Ah, interesting! I'll try to add a fullscreen option in later updates, I'd imagine that it'd fix cases like this by using the monitor resolution for figuring things out. Glad you found a fix, though!