I'm also having this issue with this quest on Xbox One, completed it fine but it isn't registering as complete in game and the old man is dead so can't try and redo it. It's the only world event I'm having trouble with at the moment and will stop me getting the "Completionist All The Way!" achievement.

This also happened to me on Xbox One, the old man threw himself off the cliff but the mission didn't complete. When I go back to try and redo it the boxes respawn but the old man is still lying dead on the ground at the bottom of the cliff.