ManuCH is right here, the iPhone has a very different opinion of what 'well lit' means, and it uses the main camera and digitally zooms in because it believes that'll be a better picture. It's not a bug per se, just Apple being aggressive in their usual philosophy of 'knowing what's best' for the consumer. The main camera apertures for all the iPhone 12s got widened to f/1.6, which should allow for 27% more light, and the software seems to be biased towards it because of this. So third party apps are your best bet to force what you want.
Even with the right camera, iPhones' computational photography tends to smudge out finer details in an attempt to reduce noise. This is noticeable when you zoom in, but likely looks 'cleaner' if you're just scrolling through social media (which is the primary audience Apple is adjusting for).