I’ve experienced multiple occasions where it lets me do the execution animation on an opponent that’s been ledged. It doesn’t register as an execution for the opponent and they can still be revived, but it gives me the renown for an execution. So it’s a technical glitch

Edit*
Actually in this video it did register as an execution, although it’s happened to me not resulting in the opponent being executed even though the animation has gone through.

https://xboxdvr.com/gamer/the-sun-danc3/video/64815665