I think I see something on my 6S+ if I really focus in on the spot for a certain amount of time and only at a certain brightness. Since this isn't a normal behavior in how I'll use the phone, it is a non-issue to me and not something I would obsess over and play the lottery with swaps. If it was an obvious and blaring issue that made that part of the screen unreadable or clearly distorted, I'd have a concern.
Every iPhone owner is entitled to their own reactions and opinions with these findings and imperfections we see in each generation. I used to be so nitpicky and particular about these issues in the past as well, also participating in the exchange lottery several times.
Then, I got a little older with these last couple generations and eventually had less time or care to worry about these things and just went about enjoying the phone rather than seeking and poring over them. I upgrade every year and wonder now why I wasted so much time over these little imperfections. Time is money, let it be IMHO. The phones are amazing.