From the release notes:
Scripting has been adjusted to prevent new stacks of Hound Damage Over Time and Vulnerability effects from being added to players if the Hound is not attackable.
I assumed "not attackable" means the Hound has bugged out and cannot be targeted, as that was probably the biggest complain about HC 6 at the start. And yes it does, but apparently if the Hound is too far away from any toon, it is also "not attackable". I was in The Snitch, and the Hound spawned on me at probably the worst possible place -- in the water tunnel before the gargoyle room. Luckily it was a Frost Hound and I had Resist Cold on. With some frantic healing I got out of the tunnel and out of the water, and got ready for the Hound to teleport to me... but it never did. And cold DOT stopped.

The mark remained on me for the rest of the quest, and when we got to the valve at the top of a buttress, we could see the Hound sitting in the tunnel. I saw its red bar, so it was probably targetable, but we left it alone.

I do not know why the Hound did not teleport, but the above means that the truly horrible griefing some people engaged in (enter dungeon, wait for Hound to spawn, step out, everyone inside dies of DOT) is no longer possible. Thank you Devs!