If the HP cap had backed out, you'd expect IP to rise. As you back the HP seat out, the spring has to undergo more compression to seal, which requires more IP behind the piston head.
Same with seat groove after many cycles. If the shaft has to travel fractionally farther to seat into the newly created groove, once again more spring compression is needed and the resulting IP goes up, not down.
Which seat did you use? Normal (X), Low (||-), or High (|-)??
With normal or low and no shims, I wouldn't be too surprised with a 100 psi IP, though as has been mentioned, it's a touch on the low side.
If it's locks up okay, just shim it up.
Do you have the good red spring?
If you were a little heavy on the lube during the rebuild, piston motion can be a touch sluggish, which allows more time for IP to rise. As the lube wipes clear and the piston movement gets brisker, there is less time for IP to rise before it seals. As a result, the IP drops. That's my best guess for a non-worrisome cause of the problem.