1. Valheim has always used Steam Cloud like most other Steam game, WHEN it is turned on in the global Steam settings. It is highly advised to turn it off. Specifically with the way IG is currently handling the world save.

2. IG DID change the way the world saves, and they also introduced a new version of the Unity engine, which might've led to the wave of world resets we now experience.

Mind, the majority of people have no issues, but for the unlucky few that do get hit with this, it is a real killer, especially since most people do no proper backups.

On top of that, they changed where the game saves stuff, with no recourse ingame to recover files that where stored locally to be uploaded into the cloud, when Steam Cloud is turned off, which is, IMHO, an absolute no-go.

3. Backups, backups, backups. It can't be stressed enough. Don't have backups? Your fault when you lose data. The game save system can be as crappy as hell (and Valheim's world save borders on the unusable), but it is YOU who is responsible for keeping your important data save.

External hard drives are cheap. Back up your ♥♥♥♥.