Hm seems like they are still having this issue. About a month ago it was either valve or COD that put out an update and it broke the game. Never tested regular multiplayer because of the hacking problem they’ve got. But anything zombies, the game would just free and crash.
If you go normally into your Windows boot. And go to your recycle bin. Do you see a SteamOS and EFI Files in that bin?
I had a similar issue a month ago where it booted fine but for some reason my system moved those files into the recycle bin and rendered my ability to boot into SteamOS obsolete. Even though it gave me the option to. I would check this and see if this is your cause