It happened to me yesterday when I tried to run around the room to heal myself. But he was completely under the ground, I saw only his healthbar and after few second he just disappeared. In addition to that, when I engaged the fight with bow, my arrows pass through the boss without hit him.
This happened to me also before Build 100728/18311. He attack charged into the rocks behind the tablet and sunk into the ground. Could kill him and got the achievement, but could not loot him. EU server #1052
same happened to me couple days ago. I was able to get him out of the ground by getting him to run near where the stone tablet is and he walked right up like a ramp was there. Then I was able to kill and loot
I had this problem multiple times with the undead dragon…
Have to go really gentle on the old thing, or he will vanish into a wall and be trapped outsde of the map… sometimes he also is submerged in the floor, and is defenceless in that way…
but he might just be a coward…
I just encountered the same bug. After he fell, I walked over towards the tablet and was still able to attack his head. After he died, I guess his body stayed below ground as I could not see it. I logged out and back to see if it might pop above ground for me, but it did not.