I also found it deleted my previous saved files if i try to save in a file with an already save file, but it don't save at all (playing in normal mode). It deleted also the new file I created to save the new game, and didn't allowed me to save in any other save file. I tried to save in top of another save file and it got erased the next time I went to play.
EDIT: I tried it again. It just let you save once. The next time you try to save, you just can't do it. I saved before the cave, and tried to do it after coming out of it. I didn't activated the invisibility spell, so that skill is not the guilty this time. And, to those who are asking how I passed the mine without the invisibility spell... the muffin boss from the tutorial dropped a chainsaw. It drops the MATK to 1, but increased the ATK to 100, and it attacks twice. Slimes just endures 1 turn, and the creatures of the mine endure 3, at most.
EDIT: I tried it again. It just let you save once. The next time you try to save, you just can't do it. I saved before the cave, and tried to do it after coming out of it. I didn't activated the invisibility spell, so that skill is not the guilty this time. And, to those who are asking how I passed the mine without the invisibility spell... the muffin boss from the tutorial dropped a chainsaw. It drops the MATK to 1, but increased the ATK to 100, and it attacks twice. Slimes just endures 1 turn, and the creatures of the mine endure 3, at most.