After completing a couple Story missions in the DLC three 3 missions that require you to kill 3 targets in restricted areas will appear on the map. These missions appear to broken, if you die while trying to complete them is good chance you will corrupt your progress and the order you do them does not matter. I recommend you kill all three captains without dying. I completed two of the missions and during the final one after I killed one and then a second captain the counter for the captains killed did not update and still said 1/3, I killed the third captain and it updated to 3/3 and the mission completed and the achievement unlocked. These missions broke TWICE for me so be very wary of this as you will have to start a new game each time to attempt it. Do not be afraid to run away, I recommend just head shotting with a phantom blade then escaping.
EDIT: R0Y4LEwitCHEESE has said that killing other soldiers in the restricted area will eventually fix this issue so no need to restart if it occurs, so try that first if it does.