Not sure if this is helpful for anyone but I randomly managed to get the quest to conclude. I just kept reloading a save game near Cassius’ shop and then went to the drop point, each time doing a little thing different (switching quests, marking this one or not, driving there straight, walking) and one of those changes apparently did it. I think it was ultimately due to me first quickly opening the drop point shop before closing it and taking the reward but I cannot be sure as I changed multiple things each time so it could have been any of the other changes as well.