Dark Agartha #8 is inside one of the dropped-down segments of the "donut"-looking arena with the superheating meteor mechanic. First one on your left from the start, for me, not sure if that's partially randomized.
The legend's disappearance had nothing to do any exploit, and has in fact been fixed up in today's patch! You can now get Samhain 2018 #9 in the laundromat. Screed finally put that legend back where it belongs. :)