Monster Hunter World: Iceborne Is Deleting Some Player’s PC Save Files
The issue appears to be affecting those who haven’t logged into the game for a while, leading to speculation it could be related to an older save not liking the newer version of the game.
Whatever the cause, the problem appears when players are prompted “Failed to read saved data for Monster Hunter: Iceborne. Create new save data?”, even though they’ve got old save data in there. Clicking yes to the request, which a lot of people just casually agreed to, writes them a new save file, overwriting the old one.
To avoid the problem yourself, you have to quit the game and when asked “do you want to close without saving”, say “yes”.
You’ll find some folks in this Reddit thread and this Steam guide with tips on how to potentially recover your file if you’ve been hit, but otherwise, if you’re one of the unlucky ones then your last recourse is either praying or hoping Capcom can do something about it.