I actually have the same defect that you're having but I consider it too minor to go through the lottery game. I have a feeling that the uneven backlight is pretty common as I've seen a few users mention it here and there. Mine is not very visible unless its on a white background especially when the brightness is turned down. On brighter settings its less visible unless I look for it.
Anyway, back to your question, I think you will be able to get a replacement if you find it too annoying right now. I believe they have screen replacement for the device already as there was a user who cracked his 6s Plus on the first day and ended up having to pay for the replacement of the screen.