I have a brand new G19 Gen3 that had this issue as well, it's called Brass to the Face/Erratic Ejections.
This solution tooks weeks of research and a month of trial and error. All the "Glock Armorers" I asked here on calguns said it was my ammo/recoil spring. Which is/was not the issue at all. I wanted to figure out the issue myself without sending it in to Glock.
THIS isn't a Definative Answer, since Glock refuses to admit this issue. But it is my answer since it stop BTTF immediately for me 110% improvement. A rare few who have done the same said it made it worse. Here is the basic run-down.
G19 Gen3 are a hit or miss when it comes to being in spec or not. I/we found out the hard way, constant BTTF on my new G19. What the issue is the old/sharp 336 Ejector and out of spec #3 Dipped Extractor made out of MIM. Thus casing weak/erratic ejections.
I replaced both parts. I bought a Gen4 9mm Trigger Housing #30275, and took the updated #30274 Ejector out from the Gen4 housing and installed it in my stock Gen3 Trigger housing. I also ditched the old dipped extractor for a #4 Non-Dipped LCI OEM 9mm Extractor, which I think is Carbon steel/in spec.
Gen4 Trigger Housing is like 8$
#4 9mm OEM Extractor is like 18$
Fixed my problem immediately, all spent casings now eject 4 o'clock all day.
Any questions feel free to ask away.