The last boss can de-spawn and never spawn again if you’re out of the room. Had to do the dungeon again to kill it.
*Also seems with how the quests work [Source of Power] ask you to go back to the dungeon after going to Tarren Mill as Horde for only a 300 xp reward, not quite sure it makes sense.
Just happened to me as well. I remember this being reported on the beta as well.
Just happened to my group.
just happened to my group too 
I just did this twice. First time portal to the final boss bugged and never spawned. SO I had to get a whole new group and then the final boss immediately despawns on pull. Bro the bugs just wasted three hours of my time! Gotta fix this man
had to do twice