"Brass to face" is a well documented problem for many Gen 4's. Seems that the extractor design was changed from the Gen 3's. My Gen 3 has a "throw too far" issue...it's constantly flinging the cases more than 10 feet...
My guess is the slide on some is out of spec. I tried extractors from several other properly functioning 9mm Glocks in my problem one and still had the same issue. I also used the 'problem gun' extractor in several properly functioning Glocks and they ran fine.
This would also explain why Glock didn't recall them---too much money to replace all the slides; sending out extractors would have only cost them a couple bucks per gun---I'm pretty sure they would have done that to 'solve the issue.'