https://www.youtube.com/watch?v=ruuP76o-bYQ&feature=youtu.be
I am encountering the same issue, the oem front flex is tied to the board but I even attempted to by pass this using reman glass to test just the LCD portion. in the video it is shown that I attach the assembly to a separate piece of glass. it functions with the OEM LCD but not the aftermarket. upon further testing I am encountering that this not only happens with aftermarket. but from oem to oem swap as well. keeping the home key and ribbon paired with its main board just swapping the LCD assembly. still results in no auto brightness function.
can not confirm but at this point I am reduced to believe they have paired something in the display itself to the motherboard