Screen defect on Dell Venue 7840

So my new Venue 8 7840 superb OLED screen is not so superb with a gray background; on my tablet at least it shows a very noticeable gradient from the top left (brighter/yellowish tint) to bottom left (normal). On the following picture you can see very clearly the contrast difference with true black and very dark gray, at the top of the screen and then at the bottom:

I'm pretty sure it's a defect, but I would like someone to confirm this, and also any indication on what to do next would be greatly appreciated!