Iphone 6 64gb slightly visible square in the middle?

Question

iPhone

Good evening. There was a problem. On the white color, a square in the middle is slightly visible if you look closely. Yesterday I bought it (iPhone 6) in principle did not notice anything like that.
The seller himself said that someone - does not know who changed the back panel because they do not match. Please tell me what it could be. Is it worth it to immediately return it or is this the norm?
Thanks in advance

Answer 1

I don't see a square

Answer 2

It looks like there might be some kind of sticker on the back of the display that is pressing on it.