Just did it 2 min ago 23:29 server time, 2/14/2018.
Engaged the last boss, and it proceeded to aggro all the upper welps followed by the boss vanishing instantly. We killed the welps and the boss didn't come back or anything. We tried to all port out and in to see if it would reset the boss and that didn't work.

Anyway, just wondering if anyone else has experienced something similar? If so this sure is an interesting new bug and I have no clue as to how it could have come about.