i have seen at least 2 different raids killing them and there was no loot (no yellow dots). i have seen the WQ pop too. i have gotten there and found them up and not engaged also with no yellow dots.
what does matter is if they are showing on your mini map as yellow dots. keep joining other raids until you see the dots. it is also possible that like some other world quest bugs i have seen in the past, leaving the zone and coming back makes the quest active. sometimes it took leaving and re-logging and then going back.