Undead Dragon in Arena clips into floor and stays there

When fighting the undead dragon in the arena, after a few moments it submerged itself into the floor with only its top part sticking out. It just stood there and let us kill it after that, then the body flung itself out of the floor and catapulted itself across the chamber, where we were able to skin it.

Game mode: Private PVE Server
Problem: Bug

Repro steps:

  1. Walk into Arena
  2. Enjoy the sights
  3. Click on the green stone and spin up the voice calling you a slave and telling you you will fight
  4. Dragon walks out, go fight it.
  5. After a few moments, it disappeared into ground while a couple skeleton archers were helping it. It stayed in the ground after that with the top of its head and back visible out of the ground but that was it. Was killable but did not fight back.

I saw this too. It started to leave and got stuck inside the stairs when I went after it.