When entering round 3 often the enemies will be agroed from people's pets returning to the spawning bay. If the first round of enemies are killed prior to stepping outside of the door and activating the boss timer (which seems ridiculously short lately) it won't count as 1 of the 2 rounds of enemies to defeat and you won't be able to lower the shield and complete the mission.
This means everyone has to bail and get 0 marks.
Also can we close off the turbolift door in the spawning room? I've been fleet mark leachers hide in there and sit afk the whole mission.