Sometimes cases can be the culprit with causing the paint to come off the aluminum. Especially if it's like a silicone case or a cheaper third-party case, it's been speculated to believe that it's a reaction with the case and the aluminum causing the paint to separate.
my iphone 7 matte black is chipped enough and it has a lot of blots on the back. I used a case made by HAMA. Bad decision ever. From now on, i will use my Iphones naked.