Oops, sorry - Itch bundles up notifications for comments if several of them pop up at the same day so I didn’t notice this one.
Are you sure about the cheating or save editing? Last three times someone reported a bug from Yorna it was a case of ”ok, ma~aybe I edited saves to get some gold” and ending up screwing game logic. Since caches were around back from the middle of the game’s development, I find it hard to understand why other players didn’t encounter the problem.
What ”restart the game from a save file” means? Did you transfer your save files from an older release into a new one and continued the story this way? The transfer can’t affect anything if you copied the save folder as I mentioned in the FAQ section and most posts with new versions.
To sum it up, all caches were already dug up until you reached the floating island and you are absolutely sure you never did it yourself? What game version are you using?
Mithril is used in your castle’s lab for crafting and enchantment. The hole is just a hole.