Thanks for clarifying. I stand corrected when I said you should report this in an appropriate way.
As your described, it appears the proper way this hidden quest should have worked is that only one set of mob (not 5) should appear be it one person or a party who did the quest, right? If so, it indeed is a bug.
As @maximiliancornejo said, it’d be best if you could include a video and/or some screenshots. Then we can summon the staff.