Kill Captain with Rope Swing

[Mynock]

Sequence 10, you must use the rope swing to kill the ship captain. I've successfully done it 7 times in a row now and it refuses to complete the optional objective for doing so. Literally the only thing stopping me from 100% sync in the game. Is this a known error? Any way around it? I've spent longer on this single mission than I did the rest of the entire game trying to get this objective to complete, and I'm doing it correctly, I've watched countless videos of other people completing it on console versions, so I know I need to assassinate him from the air after letting go of the rope swing, which I have done 7 times now...

[Quinn]

for me edward doesn't even want to kill him, i press the button while i'm on the rope and he refuses