Not sure if this is known or not, but there is still some missing npcs issues.
Please look at my mission Missing from the Miror by @Deffdog
In the second scene, I have several npcs chatting around a room. I am given instructions then need to return to them to move forward. Now the contact I had returned and I was able to be complete the mission, but half of the npcs disappeared. At the begining I had A pair of klingons with one beating on he oher thats on the ground, and I come back to that klingon beating down on he floor. The other klingon was gone. Thats just one example, there were other missing.
My mission might be a good guage to see when this is fully fixed.