Last year I had problems with my iPhone 8 (fast draining, random shutdowns), and in spite of the battery reporting high quality, when they ran it through diagnostics at the store, what they saw confirmed my experience and I was offered a battery replacement under AppleCare, and because they didn't have a battery available immediately and my AppleCare was running out in a few days, they did a white box swap (the fact that my phone was in pristine condition and could become a white box swap itself probably made that decision easier for them). Also might have helped that the person who assisted me was a senior tech and could authorize a white box swap herself instead of having to get approval.
So that 91 percent isn't the only thing they look at. But I don't know if your "fast draining" experience is like mine was. I would go to sleep with a nearly full battery and wake up with it drained. When put on the charger, the "drained" battery would charge very fast. The issue wasn't so much that the battery capacity was bad, but that it was reporting bad data to the phone leading the phone to believe it was drained when it actually wasn't.