I would sure be thinking about the HPOP. The pressed in ball bearing can pop out and the HPOP will suck down the low pressure oil pressure.
Before going to that extent though, be sure to pull, inspect, and clean the regulator on the low pressure oil (it is by the harmonic balancer).
You probably should verify that the oil filter drain valve will close when you push down on it (ie push down on it and crank and see if it fills in 3-4 seconds, and stays full until you let off of that drain valve plunger).
And just because I am usually detail oriented, you didn't confirm that the oil filter cap is also OEM (it should only be 1" tall when measuring from the filter housing to the bottom of the top hex nut).
Oh, and has the HPOP been replaced before?