This same thing happened to me. I have now done this strike 3 times to finish this bounty, and even had people with me finish the bounty and receive their exotics no problem, but mine still only says I've killed the Warden. I did it with my friends specifically for the Arc warden twice, and once with strangers doing both Solar and Arc. I really don't want to have to keep doing it and hoping it works eventually. Please help, Bungie!