I have grapple shot another player throughout multiple games, sometimes detaching the grapple shot before I fully reach the target, another time where I fully reached the target, but didn’t kill them, and another time where I grappled to a target the full way and killed them with an energy sword from the grapple animation. Not a single one of these interactions fulfilled the grapple shot another player challenge. I think it is just a bug as the new challenges rolled out today, but this is one of the new bugs with the update.
Just trying to help but they updated weekly challenges today and removed difficult ones. Are you sure you still have that as a challenge?