Try the problem mags with the upper off and see if you have a problem with them locking in place.
Have tried that and the results seem consistent with previous observations. The pmag will not lock in whatsoever, even empty and with the upper removed, simply doesn't engage. The "bad" mil-surp mag locks positively with the upper removed, but then it also seems to lock when loaded and with the upper attached, just drops out upon firing.
Inspecting the mag release doesn't reveal anything that appears to be amiss.