[2022] Multiplayer Game auto saves overwrite single player ironman game

The game doesn't use save slots, it uses the Windows file system. So if you write a file with the same name, it will overwrite a previous file with that same name, yes, as happens with any Windows app.

There is a backup which is Documents/Paradox Interactive/CK3/last_save.ck3 , did that retain your lost game?  

I've been re-thinking this situation. FYI Ironman doesn't actually do autosaves as such; rather it does automated forced saves at certain events and intervals. Its file name is the one you give it when you start the campaign, which really should not be the same as the MP autosave filename. In fact it is by default xxx_1066_Ironman.ck3 , it then just overwrites that file every save (rather than making separate autosave files).

So it is really hard to see how your MP autosave could possibly have overwritten it! You'd have to have chosen a filename for it that was identical to the MP autosave file name, which I'd assume you did not.

So I now have no explanation for the loss of your Ironman save file. It is definitely not in the /save games/ folder, nor on the Steam cloud?  

The only thing I'm aware of that would change an Ironman game to a non-Ironman one is loading it with mods enabled - is that what you think might have happened?