that IS a bummer and I'll look into it after the contest has ended. Although it shouldn't have overwritten you yet. The way autosave works is that it saves every time you enter or exit a dungeon, house, or cave (it also saves after getting a ring). So if you quit right after beating the boss, then you quit the game, you should be at the beginning of the dungeon as that'd be where you last autosaved. Assuming you didnt leave the dungeon.
Single slot save was the only way to get auto save to work. Although now I'm seeing that multiple save slots might have been possible but only slot 1 would be auto-save. Might patch that in post contest also. As always thank you for telling me this, and thank you for playing my game!