@gamingonlinux I had this problem on both my desktop (running Steam on Silverblue) and Steam Deck...
And found a solution:
Remove the prefix. Launch again and it'll build a new prefix and it'll work again.
This fixed it on my desktop and Steam Deck. Protontricks can find the prefix.
It feels like they usually goof up these Halo updates. 🫤
My wild guess is that it needs a newer version of something (probably EAC?) and it isn't updated? But redoing the prefix sets up the latest version? 🤷
@garrett my test was done on a brand new install