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In my experience with a couple of rifles that did this, primers actually back out on firing when there is generous head clearance. That is very, very common in pistols that headspace on the case mouth.
It generally takes something more than 25,000 PSI or so for the case to slide back against the bolt or breechface and re-seat the now spent primer. If it's a normal-pressure rifle round above about 45,000, the primer can balloon out before getting squished, resulting in a false "flattened primer" situation.
I've had plenty of fired rounds with backed out primers showing no pressure signs. Radius of the primer "corner" almost unchanged from before firing. That was BEFORE I fixed the over-sizing situation that was wearing the cases out in 3-4 reloads (the full power ones that did not leave a high fired primer).
Good note on the firing pin hole resulting in false "cratering" pressure signs.
For funky charge drops, I recommend the dryer sheet followed by rubbing graphite on everything with a Q-tip. If all else fails you could also tap a small screwdriver handle on the measure every time it is in the dump position. The old Lyman 55 Ideal powder measure had a little steel "door knocker" for just that purpose. Just do it the same every single time.
Be careful, but carry on. The biggest danger I see in non-defense loads and charging challenges is a lodged bullet too far in the bore, followed by a live round. I know one guy who did that and bulged a nice barrel, in a spot that compromised the semiauto's function, and I've seen a few revolver barrels similarly afflicted long after their respective events.
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