I was at the key flame at Bleak Sand in Hallow Fall killing a group of mobs to the left of it when I slowly started falling through the floor. I was playing my Demon Hunter in her metamorphosis for and had used Fel rush when this occurred so I’m not sure if that specific course of actions was the problem or if the floor is bugged over there but I would definitely be wary of it.
TWW seems to just have a massive DH/Fel Rush bug. I’m getting this issue several times a day.