So it seemed at first that daz88 was on the right track - upon closer inspection, there was a very small nick in the surface where it meets the oring. You can see the brass coming through where the galvanized finish got chipped off somehow.
Took DiveBuddyChgo’s suggestion of switching out to the black O-ring. Also, did some research to find that silicone grease is a DEFINITE no-no for tank O-rings, especially if you’re putting EANx in there, because it is flammable I guess.
That all seemed to fix it for a while.... Until just recently when the problem re-emerged! I probably did 12 dives in the interim, and only just now failed the buddy bubble check. I had a moment, so I changed O-rings again. Still a little bubbles. Did the dive anyway, no problem, but still a little stream at the safety stop.
One other interesting thing is that I turned the regulator over, just to check that it would spit bubbles out the /opposite/ side, and it didn’t! Same spot on valve.
That leads me to think (as I saw a little when I was changing the O-ring) that there are possibly some dried salt crystals in there behind the oring allowing it to leak out. Not a problem of the reg anymore at all! The old problem was replicable on other tanks, but I haven’t tried my reg on another since it re-occurred.
Doesn’t seem to be dangerous, so I’m going to keep an eye on it and see what ultimately fixes it. The faber blue steel valves seem kinda cheap and not very "meaty".