According to STO Gamepedia (https://sto.gamepedia.com/Mission:_The_Khitomer_Discord) the survivors accolade is bugged as you can still only rescue 14/15. I also have the same quest problem, stuck at completing "head upstairs"
Same problem occurred for me in Cure Found (Advanced) [Ticket #375923]. After waiting for leaver penalty to expire, next attempt was successful. Leaver penalty due to a bug seems unfair, although I realise the penalty is automatically issued by the system.
So the latest Endeavour task can only be completed if you have a Matter/Antimatter Specialist with crafting skill. Fat chance of completing that - thanks a lot PW