Mobs go unresponsive after mission completion

[srmalloy]

In the mission 'Stop the gang fight at the Portal' from Haley Phillips, if you defeat the two bosses without clearing the mission, you'll see NPC chat like "Let's get out of here", and mobs running for the exit -- but not all the mobs will run. The ones that remain will continue standing or walking around, and become unresponsive -- you can attack them, and they won't run or aggro in response.

[Wavicle]

I saw this exact same behavior the other day, in the version of the mission from Trevor Seaborn.