OAL WAS THE PROBLEM!!!
The initial spark from the primer flashed over the forward placed powder charge causing delayed ignition. The lack of firing pin hit was the result of the firing pin/hammer rebounding before the ignition allowing internal pressure to flatten the pin mark.
I’ve seen it before with deteriorated powder. In old .38 cartridges.
Ha! Funny you should mention this.
I brought up what you've described (S.E.E.: Secondary Explosive Effect) in a thread on another forum recently as a point of discussion only, not proof of a incident being discussed there, and was chastised for bringing up an "old wives tale"; a phenomenon often repeated but never proven (for the record, it does NOT appear that it's ever been proven under laboratory conditions)
Like I said on that forum, I've been around long enough to know to "never say never".
Without knowing all the specifics of our OP's particular load, I think it's a bit premature to say that's what happened.
Personally, I think you would still see some kind of evidence that the primer was struck by the firing pin.