Of course it is hard to verify this as a user, but I finally gave in a purchased a 400% eare booster due to it lasting for 6 hours while farming Super Hard Advanced Quests. Started after Quna concert.
Ever since I used it my drops have been significantly worse to the point that I think I got roughly 1 12* and maybe 5 10*s through 2 whole cycles of Super Hard quests. The drops in the UQ that followed the concert also seemed horrible, and we also triggered the 300% rare boost from the jack in the box on top of it.
I will also be submitting it as a ticket so they can check the modifiers on their end, but I definitely haven't had this few 10* drops ever playing Super Hard advanced Quests, especially on +50 risk. (Of course I also didn't get any 13*s or Saiki, but that's not quantifiably the boosters fault).
Anyone else have any anecdotal experiences with the AC Rare?
EDIT: Just to add to the circumstances, I overwrite a 250% Booster with it.