I can't really see them not replacing that. Unless there is some drop damage somewhere else in the phone. But if your phone hasn't been dropped that's a manufacture defect.
They over tightened the screw during assembly. Take it back. It's a defect.
I just think it is interesting that it is so close to the screw.
I doubt it. Aluminum is soft and those screws can't have much torque applied on them in the first place