Late last year my wife's phone was damaged by a battery replacement. She had the 6s+ shut off battery issue and lived with it for a year, we took it in, complained, showed them how it drained and shut off before 0%, they said battery was green, so I shelled out $89? to get it replaced. In so doing, they screwed up the screen (half of it wouldn't light up) and the phone restarted constantly. Fed up with the treatment we got at this store, we went to the one we normally go to near our house and in 6 mins she walked out with a brand new 6s+ for $0 (AppleCare+). Apple eventually credited our CC $55? a few months ago in response to the $29 thing.
It happens now and then, hasn't happened to me personally. I posted the above in a handful of threads back when the battery shutoff was trending.
My vote is the phone was accidentally damaged.