hi man.
Your observations strongly suggest a map-change / fastfile lookup path issue, not a missing file issue:
cold join works
private match works
lobby map rotation fails
In other words, mp_alpha.ff is clearly present and loadable, but the rotation path appears to resolve zones differently and falls back to the generic “fastfile not installed” error.
The biggest clue is that Plutonium IW5 uses %localappdata%/Plutonium/storage/iw5/zone as an override location for .ff files. Since your storage/iw5/zone already contains multiplayer/DLC fastfiles, I’d strongly suspect that this folder is interfering with fastfile resolution during map_rotate, especially under Wine/Proton.
I would test these in order:
Rename %localappdata%/Plutonium/storage/iw5/zone to zone.bak and retest map rotation.
Temporarily remove any custom scripts, plugins, mods, and zone overrides from storage/iw5.
Make sure the Steam install language is English, then run Steam Verify integrity / repair on MW3.
If you are launching through Proton, try a plain Wine-GE / Wine setup instead. Plutonium officially supports Windows only, so Linux/Proton path behavior can absolutely be the variable here.
If possible, test the same install on native Windows to confirm whether this is a Linux/Proton-specific fastfile resolution issue.
My guess is that the error is real only in the sense that the engine’s rotation-time lookup is failing, not that the map fastfile is actually absent.