Well, I set out to make a video showing how great the ejection was in my Gen 4 G17. I've had roughly 900 rounds of perfect ejection, with lots of video to prove it on my YouTube channel.
Half way through getting my footage for this video, it happened. I started to get BTF.
I demonstrate the ejection of WWB, Speer Lawman 115gr FMJ, and Ranger T-Series 127gr +p+. Each of them ejected just fine for me previously, but by the time I switched camera angles, they all started to shoot brass into my face.
As I show you, this BTF doesn't matter what ammo is being used, and it doesn't matter how you hold the pistol. There is definitely something that has shifted with how this pistol is engaging and ejecting brass, though I can't see anything wrong with the extractor that looks like abnormal wear.
This seems to match the new experience mentioned by others here, where you get great ejection from brand new Gen 4 9mms for a while only to have it end up in your face after 800 rounds or so.
Check it out and see for yourself.
ETA: Here are the companion videos. The slow motion comparison is especially revealing.
Brass Not To Face! Gen 4 G23 Ejection Examined
Slow Motion Glocks! Gen 4 G17 & G23 Ejection Compared