I don't think Amir is shot or his chin is cracked. His punch resistance has always been average. It's just that all fight long Diaz was focusing on throwing counterpunches to knock Amir out.
The difference in previous fights is that even though Peterson or Maidana also landed flush shots on Amir's chin, none of them were really counter shots. The punch that Maidana threw in the 10th was a counter punch and guess what that one sgtaggered Amir. The same goes for that Prescott hook and that Garcia hook. All counter punches.