Went to 3 (yes, 3) Apple stores about this issue. The first one had no replacements but told me to call later in the month. Went to a second this morning, who acknowledged that my screen had an issue but also had no replacements. They sent me to yet another, who gave me a replacement. However, the new one still has a small spot, albeit nowhere near as bad as the original one I had. I pointed this out and the Genius helping me saw that it still had an issue. He checked the display phones in the store and confirmed they all had some degree of issue, but couldn't give me a different replacement because they had no more.
The takeaway: there's definitely some kind of design flaw / QA issue with the screens for the 6S pluses. At this point, they haven't fixed or officially acknowledged it. The one I have now is better than before but has one minor dark spot in the corner where I barely notice it. So I'm not thrilled, but I'm happier than I was with the last one.