yes it's a bug that comes and goes where you can't break up the bar fight. sometimes it finishes even if some are still around and you have to contend with them shooting at you while doing the devidian stuff. It seems to jump around as I had a KDF mission that did this but fortunately didn't need to kill them to move on.
try beaming out and back in (sometimes works) or reacquire after logging out as some of the others have said. (btw this was doing it ALL the time, so they did fix it right after S4)
You might try taking out all of the bar fight participants before you take care of the devidians in the bar. That has usually worked for me and I haven't had the bug pop up again...but I realize I may have just been lucky, too.