I'm sorry this happened to you. Shortly after we released the game on PC Game Pass, we encountered a major bug on that platform that caused some players to lose their progress. It turned out to be an issue with the cloud save functionality of the Xbox app itself, and not a bug in the game, as several other titles have recently experienced similar issues (such as Lamplighters League or Wartales).
As far as we know, Microsoft is still working on this and looking for a fix on their end. Meanwhile, we have released a workaround (in patch 1.0.5) that adds an additional layer of local backups and ignores faulty cloud saves. You can read more about it here: go.eremitegames.com/AtS_Patch105
In this document you will also find some custom save files we have prepared, and instructions on how to apply them to your game.
Once again, I am terribly sorry that this happened.