Oil type

The head likely lost a valve seat and took out the piston. Pretty common from what I have heard. When I bought my core motor it had been cannibalized by a couple guys who needed one piston/rod assy to fix their motors. This was a few years back when the pistons were on a very long national backorder. In the case of your builder, obviously its cheaper to replace one than all of them.

Break ins should be done with non-synthetic oil, whatever your app calls for from factory. Additives not needed because of the roller cams, but prolly wouldnt hurt if you felt the need.