I've heard quite a few people with all of the iPhone 12 models, mini/regular/Pro/Pro Max report rapid battery degradation from November 2020 to now. I really do believe many of those phones shipped with batteries from an OEM that were faulty. I have an iPhone X with a three-year-old battery at 92% health, but the 12 mini already reached 89% after less than 9 months. Both have been charged using the same chargers.
I'm not sure if they haven't investigated it (perhaps out of fear that it might lead to another service program and fines), or have and aren't admitting to it. But either way, I don't think there's any other reasonable explanation for the number of reports of quickly degraded iPhone 12 batteries.