Not possible unless there is some bug, for anyone to terraform the mine entrance or the blocks around it (the ones that show up red in observation mode). An unreinforced tunnel will collapse in a few game days. That should certainly take more than 15 real life minutes, which means something else must have happened.
Also, if it was a collapse then you should only get small amounts per dig on that dirt and not a full 30 each time. If you do get 30, then I suspect that it is server related. Did the server reset over that time? It could be that the change was rolled back (as if it never happened).