Err… Let me get this straight - you used 30th slot for a manual save and then tried to load it, right? Well, that’s why I recommend using any other slot except 30th. There 29 more and it should suffice even for a very cautious approach.
Just as I write in the FAQ post, the autosave feature will overwrite 30th slot each time you change your current zone. Besides, due to players’ requests, I made potions that change Ada’s lewd level. You can’t make an irreversible mistake in Yorna.
There’s no option to disable autosaving since it’s harmless and beneficial if you don’t use 30th save slot for manual saving.