today there was some kind of "loop"-bug in the last bossfight of the Khorne-DLC. Where you fight the Voidripper Boss(es) together with the Stormwatcher Grandmaster.
As you can see in the screenshot the game doesn't recognize the win and the objective pops up again where you pick up the grandmaster after he sends his Astartes away to fight with you. Than the bosses spawn again and so on. In the end you have 3 Grandmasters with you and the mission-goal can't be reached so you have to restart it.
After restart the mission worked properly