I've got a relatively new 3rd gen Glock 23 (Purchased in the past 3-4 months). I've got a 9mm barrel as well as a 40s&w barrel for it and I believe that it's the stock extractor in it.
9mm and 40s&w brass ejects erratically most of the time. I set a tarp out last night to catch most of the brass so I could reload it and noticed that maybe 15% of the brass ejected directly back over my head. Is it correct that by ejecting erratically that it could be a sign of poor extraction? I need to put more 40s&w through it, as most of what I've been shooting is 9mm with the 40 extractor. I've had one FTE but it was while my dad was shooting it and I have no clue whether it was limpwristing, or the 15 year old ammo we were using.
9mm and .40 extractors are aren't the same.
That could be one of the issues.
My Gen3 G27 had erratic ejection, everything was stock and I tried multiple brands of .40 ammo. What finally fixed the ejection problem was to install the new 28926 ejector that's being used in Gen4's. I had to pull the Gen3 trigger housing out of the frame, pull the old 1882 ejector out of that trigger housing, and insert the 28926 ejector into that trigger housing.
The newest Gen4 trigger housings sold by Glock have the new ejector in them, but most of the retailers probably still have the old version with the 1882 ejector. No retailers have been confirmed to have the new version with the 28926 ejector yet. If you were to get a Gen4 .40 trigger housing with the 28926 ejector, you'd have to remove that ejector from that trigger housing and install that ejector into the Gen3 trigger housing that's in your gun. If you continue to experience erratic ejection with that ejector, it means the extractor doesn't move freely enough or is otherwise out-of-spec and needs to be replaced. Keep the new ejector in the gun and replace the extractor.
Alternatively you could try simply replacing the extractor, but it's possible you could just end up with another one that's out-of-spec.
I would not see erratic ejection when shooting 9mm out of a .40 gun as a bad sign, I'd only see it as a bad sign when the gun is shooting the caliber it was originally chambered for.
One of the symptoms of the erratic ejection problem is a triangular dent/scrape at the case mouth of most of the spent casings: