Well, I am stumped, if the gravity is not dropping in the bottle than I don’t think it’s a brett infection. I don’t know enough about microbiology to say if other spoilage organisms metabolize in such as way as to not affect the gravity.
Would a point drop or more in the bottle cause it to foam over or constitute an infection?
seems unlikely with only 2.9 oz sugar to start with, however 1.001 is what you would get adding about 1.6 oz sugar to 5 gallons so it is effectivly increasing the priming sugar effect by ~50%. I mean it seems odd that given that low a FG that it could drop any further without some bad a$$ yeast helping out but I really don’t know.
Infections can be strange. I think there’s something going on with your beers besides just overcarbonation. I’ve gone to 5 volumes without gushers. CO2 comes out of solution much more quickly if it has nucleation sites. If the 5vol beer is well-settled it won’t foam over, if the yeast is up in solution, it foams over quickly. So I guess it’s possible your infection isn’t actually changing the gravity or carbonation level, but the bugs can stay in suspension, creating nucleation sites and foaming over. Just a WAG.