I have a theory about this.....
When you "dig" the invisible mob on the ground you don't defeat it. You do make it move to a new random location, but its not defeated. When you "dig" the mob numbers pop up showing damage like a normal mob, and I've even 2 hit combo'd the ground before by getting the dig spot right away again. I believe that each maps dig spot is one spot that has a set amount of HP and "runs" when you dig it, and that in order to get anything from the ground you have to "defeat" the dig spot.
Just my thoughts anyway what do you think?