I know I'm a bit late to this topic so if what I'm about to say has already been noticed then ignore it but I've noticed that on PoA it doesn't always do fall damage registration. So even if you do get the assassination because there was no fatal fall momentum being "recorded"(for lack of a better term) in-game then the challenge won't register. The way I've noticed this is jumping off the cliff, not trying to get the Elite, & just trying to see if I take any fall damage upon hitting the ground. If I was completely fine then I knew the challenge wouldn't unlock due to, again, no fall damage momentum being recorded by the game. So, with that being said, it's not necessarily the challenge itself that's bugged. It's the fact that "fall damage logging" by the game's code is what's screwing people up for completing the challenge.
[Locked] Because you begged