Presumably Glock, the FBI, and Indy did a fair amount of testing, that included a variety of holsters.
I will bet ten, my whole month's allotment of PF wagering dollars, that the problem has nothing to do with holsters, and has everything to do with the slide lock -- possibly the design if something changed with it, how the problem pistols were assembled by Glock, or something done with the pistols locally, post delivery. Anyone else want to get in on this bet?