If it does this and you log back in and exit you will save over your backup and you're done. Happened to me except I just died. No crash. Everything went poof. I logged in and out before I googled a possible fix.

https://www.reddit.com/r/valheim/comments/vssxdi/how_to_restore_most_recent_working_world_save/

The fix apparently is on the beta build right now but not live.