Q:?
Did you use new brass on your first series of load workups, and then reload the brass by neck sizing it to do your later loads?

A neck sized piece of brass is going to fit the chamber of the rifle alot tighter than a new piece of brass. A new piece of brass is likely to leave a bit more headspace in the chamber where the brass needs to expand to fill it out. When this happens, the base of the case is pushed backward fairly hard and can flatten the primer. The force pushing back on the necksized cases isn't going to be quite as high as the headspace will be less, and you won't get the false-positive high pressure signs of the primer flattening.

This would be my best wild -ss scientific guess as to what might be happening.