If you are having trouble playing with friends/folks on windows PC, you need to copy windows versions of the files and place them into the compdata folder for Halo on your deck/PC. After that we were able to play without desync.

Note: you may need to delete+reinstall compdata before taking these steps. I just did a fresh install to make it easy.

• The windows files are located at: C:\Windows\System32\ucrtbase.dll and C:\Windows\SysWOW64\ucrtbase.dll. Keep track of which is which, the files are different.
• Once you've copied the files to linux/Steam Deck, place them in steamapps/compatdata/976730/pfx/drive_c/windows/system32/ucrtbase.dll and steamapps/compatdata/976730/pfx/drive_c/windows/syswow64/ucrtbase.dll.

I found this information in the following post: https://www.reddit.com/r/SteamDeck/comments/11dftq1/fix_halo_mcc_coop_between_windows_and_linuxsteam/

Original discovery by Fractal: https://www.protondb.com/app/976730