Hi, I have the same issue with my 6s, I bought one in December 2015, and put it in an Apple leather case almost as soon as I got it. A few months later, I decided to take of the case, and saw that the whole back of my phone was chipped. I was really frustrated by this, so I took my phone to the Apple Store, but they claimed I must have done something wrong, so I paid $200 for a replacement, only to find that a few months after getting the replacement, the back is chipped again.Aluminium on my 6s Plus has been chipping on its own especially at the edges. It's appearing at the bottom of my phone all along the antenna band and now it's appearing at the top. It's also appearing around my pentalobe screw holes, lightning port and headphone jack.
I seriously think it's a manufacturing issue. Some problem with the 7000 series aluminium that's causing the edges of the aluminium to be very very easily chipped. I used my phone with a cover since Day 1 and yet this happened. I used my 6 Plus pretty much naked for the whole year and yet it hasn't chipped even though it has softer 6000 series aluminium.
Do any of you have this issue?