This not a bug so much as a error in game desighn, but once you put items into a stack, you can't seperate them again, for example, you can not seperate five coal into a stack of three coal and two coal.
cool! Although, I was playing around and found a way to do it! By dropping the stack then picking up a single item with tongs, then pick up the single item, and hold it with your cursor while pressing e to pick up the dropped items, you can seperate a stack, I know it's complicated, but now I can have my first ore mined seperate from all my others!