I clean my guns after each use with CLP, a bore brush and a bore snake. The chamber and barrel were clean.

The rounds appear to be seating properly in the chamber. I don't think it was out of battery.

The piece missing from the head is 90 degrees from the firing pin mark, which is consistent with the geometry of the bolt. The extractor may have hit the rim and caused a defective round to break.

I used a brass catcher, so I was able to inspect the other spent casing and there were no defects.

I am going to go with a defective round. I will watch it closely the next time I take it to the range.