It is after market screens, we see this every year with the new iPhones. There is always something manufacturers need to work the kinks out. The 5C had bad touch issues, the 6 had frame lifting issues, 6s with force touch and now with the 7 it's the aftermarket force touch/backlight again.
We should be getting answers from suppliers. And iFixit screens do the same thing, I've tried. I've only had a couple screens from different suppliers work. It's like a needle in a haystack right now.