Don't worry, I'm a save-scummer. I save before every map transition, among other things, so my most recent save was probably created a few seconds prior to the mishap. :D Since I had looted the treasure chest before exploring Fantasma Cavern, I simply wanted to check afterwards if there's something new in that area (like a ladder trigger). It's a good thing that it was me who discovered the issue that way - and not someone with autosaves disabled and no manually created save file up to that point (as unlikely as that is to happen).