Your 458 PT looks just like the 416 Wby 400 gr PT I recovered from water jugs. And the rear core was intact.
Anything mechanically manufactured has the possibility of having a flaw. Anything assembled by human hands to be more exact I think. I would think that the guide or guides are probably drawing on a single experience with regards to rear core separation of a Partition. Maybe two, but not regularly happening. It was most likely on the very toughest and dangerous game where a single occurrence could be fatal. I understand their claim because client safety has to be paramount at all times for a PH. I’ve never seen that happen with a Partition but I haven’t shot them into animals that double as 3-5 ton trucks either. But I do think the occurrence is an exception and not the normal.
I think what he meant was that if it had functioned properly, the elk would have run uphill in a helpful direction before expiring.Once had a friend tell me the Partition I used for the elk I was quartering had failed. He went on to explain a proper bullet would have stopped against the hide and not exited. I told him to be quiet and pull up on that rear leg we have work to do and 1500 feet of vertical to get out by dark.