I found the bug! It happens if you use a save created in a Patreon-exclusive version of the game in the free tier of the game; I just had it happen to me again; everything was immediately wiped back to Enchanter Level 1. I am assuming that this is to prevent people from cracking your game and punishing them, but if this is the case, it is also punishing legitimate users.
Edit: For clarity, I support temporarily on Patreon to get access to the new build, and then immediately unsubscribe (I get charged any time I want to download the new Patreon-exclusive build, but that's fine by me) because I don't want NSFW notifications in my personal email and I like being able to access my Patreon account without having to look over my shoulders/because I don't always want to see NSFW content. So I wanted to try out the new content without paying for Patreon exclusive but found out that my saves were nuked because I tried loading Patreon-exclusive saves on a non-Patreon version of the game.
As well, I can understand, assuming it was intentional, that you want to prevent people without Patreon versions of the game from cheating and getting Patreon-exclusive content, but it should just prevent you from loading the save, and not nuke it.