Well, this backs up my earlier statement: if it is a brass issue, there would be more talk to support it--it seems there IS. Bad annealing from the factory would be a quick guess, as you've already mentioned. Your loads seem to be right in line. If the same load doesn't produce similar outcomes in different brass, that pretty much sums up the general problem.
"Why is is called Common Sense when it seems so few actually possess it?"
Guns only have two enemies: Rust and Politicians.