Bringing a thrall into the Palace of the Witch Queen will get it stuck in a bizarre “swimming underwater” animation while it floats in the air, until you move beyond leash distance and force it to teleport to you.
I can try it again at some point, but I’ve seen some streams of players bringing thralls into the instance and they didn’t do this (sometimes they’ll teleport in where you’d normally expect, but one streamer had them show up below them, close to the water on the sides). I imagine this may largely depend on where the player moves to shortly after they’ve entered the dungeon.
When I entered the dungeon, I actually stopped and waited to see if the thrall showed up correctly. I suspect if I would have just kept running forward across the walkway into the boss room, this wouldn’t have happened. As it was, when I did move forward, the thrall teleported to me again and “reset” himself and worked correctly.