All those are things that can happen for sure. I believe the OP, if he says he saw blood on a shoulder hit and his brother did, then i believe him. Same goes for
Video of shot impact. If the OP is to be believed, then i dont care which of the above types of failure happened. One of them did. Also runs consistent with what many if us have seen happen with that bullet. No deer should be alive after a hit in the shoulder.