Just went to Neptune Proxima to take out my Sister lich. I beat her, choose to Vanquish her, watch my frame stab her...
And suddenly teleport outside into my Archwing, and I am told MISSION FAILED. I thought maybe it had to do with the fact that I was soloing her.
So I change it to Invite Only, try a second time, down her and return to my ship, no problem.