The dents that appear in the top round in the magazine could well be due to escaping gas.

That's an interesting warning on the ammo. I was lectured long ago about always reading the fine print, but this is a first.

If you wanted an authoritative independent opinion on the cause of the failure, HP White Laboratories does that stuff for a living. You could call and get an estimate. I'd expect about $1K minimum, but the one time we used them, we got reimbursed.