It’s “done” because the numbers around it leave no option to move its mines around. Since the mine locations have been fixed in that particular part of the game

It’s “done” because the numbers around it leave no option to move its mines around. Since the mine locations have been fixed in that particular part of the game board, you’re guaranteed that the square at 3,0 is safe. Instead, you clicked on a square you couldn’t possibly know was safe (in the middle of nowhere) when there was at least one square that was definitely known to be safe, so the game punished you for it, exactly as promised. I played a few rounds on an iPad, so I had no way to mark mines, and the game works just fine. Marking mines has always been for your benefit, to make it easier to remember where the mines are.