Xperia Z3 yellow spot

So, after my Z3 was exchanged for a new one in february this year because of the faulty usb flap, my new Z3 now has a yellow spot developing on the left hand side of the screen next to the usb port.
My retailer says this is not their fault and they won't exchange it. They can only offer a repair which may cost approx. 200 Euro for a new screen.

Did anyone of you have the same issue? What did you do? This is a known issue isn't it?

Thanks!