I have had the same problem. The quest says I have killed the warden of Oryx but not Alak-Hul. I have completed the strike twice meeting the parameters both times but no quest completion. If this is a bug it would be great if you could fix it. Update: Abandoning the quest reset the progress and allowed me to go back and kill the warden and Alak-Hul and get the exotic sword. Edit: this does not make you do every step of the quest again just send you back to the objective that says kill Alak-Hul and the warden.