... "could be the chamber."
this would be my thought as well.. having experienced exactly the same problem with 2 different PSA uppers.
Talked with my gunsmith & that was his thought. Could return the upper to PSA, let them handle it.
My gunsmith suggested polishing the chamber.. How ? -I asked.. he answered/explained...
Using a piece of hardwood dowel, 1/4" diameter (or smaller) (larger is better / stronger.) cut a length of dowel approx. 12" long.
on one end, cut a slot across the middle of the dowel. This is the end that will do the work. Can use a fine blade saw, hacksaw / coping saw, etc..I use a dremel with a cutoff wheel.
The slot should be at least 1/4" deep, as you will be inserting material into the slot.
Next get a scotchbrite pad (like this..
https://www.amazon.com/Scotch-Brite-.../dp/B000LPN3WQ)
the different colors of the pad are different "grits"..like sandpaper. Dark redish-brown is coarser then green. I've used both, the red-brown gave better results, for this use.
Using scissors, cut approx. a 1 " square of the scotchbrite material...then, insert 1 edge into the slot cut in the dowel.
chuck the other end of the dowel into a drill. insert dowel with pad material into chamber, start dowel spinning, while moving the pad forward & back in the chamber.
Don't worry about removing too much chamber material.. you won't. But the pad will wear, fairly quickly.(depending on how rough the chamber is, how much carbon is there, etc)
First pad will turn black from carbon stuck to the chamber. Replace pad when it starts looking worn / dirty.
You can add a solvent to the pad, or use it dry..will need to experiment with the best size pad you cut to fit the chamber.
When finished (or you get tired).. flush chamber with cleaner / solvent of your choice..(I spray wd-40 into chamber,) & letting it run out the muzzle
Follow with a few clean patches. One upper that I did this process to required a 2nd polish effort, before problem resolved.
The other upper required only 1 treatment (so far)
The scotchbrite orange pad "Performs like steel wool grade 1 without splintering "..so it's NOT highly abrasive.
This process worked very well for both my uppers that had the same problem you described.