It’s like a car braI work in cellular retail sales, and see dozens of well-used(loved?) iPhones every day. I definitely wouldn't say it is a widespread problem, but is something I have taken note of in the last few years.
Since the release of the iPhone 12, I have noticed that the odd device here and there will have paint wearing down at the bottom next to the speakers and charging port. Coincidentally, almost every phone that I've noticed with this issue has come out of an Otterbox Defender case which the client will always claim, "has never left the case since the day the phone was purchased."
I have never once seen this on a Pro model, which I'm to assume is due to the stainless steel chassis compared to the aluminum of the base models.
Another observation, basically every single device I've seen with this type of paint "chipping" has come from users who work in rugged environments and definitely do not take great care of their equipment. You get out what you put in.
Not any particular cause for concern, I don't think quality has been impacted in any negative way.
You trade a possible rock chip for a wear line along the edges of the bra