Hello my Ifixit family.
After having repaired quite a few Iphone 6s now I've found a solution to the most common problem with the sensor.
While extensive testing I found out that the light leakage is caused from inside of the phone. Simply adding a piece of electrical tape between the internal speaker/camera where this is a large gap stops the light from the LCD from reaching the sensor, which is the problem. (Note, this leakage can easily be tested by making a voice memo, changing the speaker in the top right so that sound comes through the internal speaker and put a tape piece/something dark over the sensor on the front side of the screen. Then take your free hand to the internal part of the screen and cover up areas until you find the leakage. :) )
I take a piece of electrical tape, double over it (so both sides are sticky, so to say) and place it over the ESD protection of the battery contact. When you place the screen down the tape covers the hole perfectly and I've yet to have a problem with a sensor flex since.
I hope this helps some one!