yeah i had many many headshots at well over 150m and was stuck at 96% (which ofcourse is a bug in itself as its either 0 or 1 so either 0% or 100%) xbox one
many times pop a cultist it ads to my longshot kills its a definite headshot and grace even calls out "nice headshot" but still stuck at 96%
it finally came out of the blue for me in a chopper of all things, dusted a pick up with a couple of peggies in it and it popped. so try that? jump in a chopper and hunt peggies or peggie trucks/pickups from high up.
it seems to be bugged for the rifle and bow its supposed to be for, in fact the whole longshot challenge for perk points thing is bugged. ive had many many 150m+ kills that havnt counted, and my final 2 for the challenge were together and no way near 150m away, way closer than id previously been shooting. I was up a rather high tower and they were just down on the road shot them both and got the challenge. its possible the height + horizontal distance to target was 150m but the travel of the round on the hypotenuse wouldnt of been close .
I also 100% know its not working properly as well, after getting it i noticed a group of 3x peggies across a ravine where shooting at some whitetails so i lined them up shot the one at their rear and closest to me and thought thats gotta be well over 150m, checked my longshot kills and yes sure enough it added to the total, so i kill the other 2x which are furthur away than the first one i kill and neither added to the longshot tally.