I encountered this bug just now, 9/29, when I joined the event (Help legionnaire boneflayer crush the Renegades) already in progress. An army of Kelith Boneflayers was assaulting the renegade hideout. This is of course in Fields of Ruin.
These Boneflayers all move out to the quest together, return to their place when it is finished, and then repeat when the event is restarted.
Maybe it generates another one every time the event is started?
In this screenshot, these Keliths ran ahead. One was continuing the quest speech behind me. When I turned around, I saw him and yet one more. He moved out when he finished speaking, with the second in front of him as well.
