My wife was playing XBOX D3 season 21 and had completed Chapter 3 challanges, the next day when she logs on she lost 30 paragon levels and two challanges. here account was the only affected.
This likely happened because she left the game by other means than how the game intends you to. Auto save does not continously happen, it happens at timed intervals or when you properly leave a lobby. If she just dashboards and forces the game to quit, she isn’t affording the game the opportunity to auto save.