Primers provide unreliable indications of pressure.
I found, after shooting dry cases in a dry chamber, in my M1a, dry loads that had flattened primers after firing, the exact same rounds but fired with case lube on the case had nicely rounded primers. Obviously case lube on the outside of the case had no influence to pressures inside and could not make pressures drop (!) so something else was going on.
To start with, if the case and chamber is dry, the case will adhere to the chamber wall. If pressures are low enough, the case sidewalls won't stretch and after firing, the primer will be found above the case head.
These are my cases, a load of 150 SMK 47.5 grains IMR 4895 fired in a SAKO rifle. I full length size my cases and try to push the shoulder back about 0.003". This load developed low enough pressures in this rifle that the sidewalls did not stretch but of course, the primers backed out.
I am of the opinion that as pressures build, and build enough to stretch the sidewalls or overcome sidewall friction, the case will slide back to the bolt face and that will stuff the primer back into the pocket. I think it is a fair guess that the longer the primer is out of the pocket the more it expands and thusly, looks flat when it is finally reseated. For my lubricated cases, case to chamber friction was quite low so the cases slide back to the bolt face when internal pressures were still quite low, thus, nicely rounded primers.
These primers are not particularly flat, but the primer has pierced and the anvil is sticking out! Obviously the pressures are excessive.
When I develop loads, now I lube my cases and examine primers for evidence of flattening. It is not 100% as occasionally I get blown or pierced primers when other primers, of the same load, are slightly rounded. I am beginning to believe that pressures vary a lot more than we have been lead to believe, and that could be the problem: given the variance of pressures, some loads of the same charge will appear OK, but others will be 20% higher, even though everything is the same, and those high pressure rounds blow primers. However, whenever I see the transition of rounded to flattened primers with lubricated cases, I am at, above, or near a maximum load. One other advantage of lubricated cases is that the full thrust of the case is applied to the bolt face and overpressure loads will reveal themselves by sticking bolt lift. Case friction between the case and chamber will reduce the load on the bolt face, giving false positive indications of normal pressures, as the bolt lift will appear to be normal with an overpressure load. Out of decades of shooting, the only time I had obviously overpressure loads that did not give sticky extraction with an overpressure load was last year. I had loads in a FN 270 Win that were clocking 3200 fps with 130 grain bullets. I had lubricated the cases and primers looked OK, and no excessive bolt lift. But, 3200 fps with a 130 is way too hot, and later, when repriming that brass, I found a few cases with loose pockets. I did install an extra power mainspring so it is possible that was disguising bolt lift effort.
The only 100% indications of high pressures are blown primers, pierced primers, sticking cases and expanded case heads. If you ever experience one of these signs your loads are way over pressure, and they were overpressure at least a grain of powder ago.
One thing you can always count on, add more powder to a constant volume pressure vessel, pressures will increase. I cannot fully explain what is going on in your rifle, maybe your false positive indications of low pressures are due to chamber polish, pressure curve, but rest assured, the pressures are not going down.
