I just try put your phone side by side with one of the iPhone 6s image from internet. It looks kind of identical. It is 4.7" to me which means it is not possible knock off version of iPhone 5s which is 4" but just your black border seems slightly ticker than Verge photo. (Unless really got people success put in the 5s hardware to 6s body include changing the LCD to 4.7")
This is very weird and I think not happen before.
The service center shall replace for you as it is their fault and during send for service they shall have generate receipt for you, that receipt shall be a proof your phone is not working as they suppose. You don't need to show the original receipt for this as this is not your fault but the service center. The service center need to fixed for you, if they can't they need to figure out the way (for example replacement).