I recently picked up a 2015 and was playing with the rear headrests today. The lock that holds them from going down appears to be on the opposite post from the one that locks it into the various positions. I had it in the all the way up position, held the release lever and pushed it all the way down.
I didn't press super hard, but I think the fact that there was some mass behind the movement, made it go past this secondary lock and go all the way down. When I lifted it again to make sure all the other functions were still in tact, I heard something fall inside there. Maybe it was this pin? The pin must be pretty weak, I didn't push it down very hard, but I had used the full travel of the headrest when pushing it down.
The visibility is a bit better out the rear window with these down. Even putting them all the way up, you still get distracted by the posts in view.