Pictured: final Dragonstar Arena boss fight setting. (Spoilers!)
Not pictured: final boss.

On our third battle attempt, after reaching <50% on the previous try, early in the fight the tank led the boss over by the corner to the right of the stairs. He seemed to glitch into the wall, and then vanished. We thought "Well, at least it was still early in the fight," and then suicided in the lava to go back to start to reset the boss fight. We entered the portal again and ... no boss waiting. We tried a few things, like a couple of more suicides, exiting the instance, shuffling a group member in and out the group, switching normal to vet mode while outside of the instance, nada. So then we 2 remaining members who hadn't rage quit posed for this screenshot.

I filed a help ticket but not sure what they can do about it. Still, wondering if this has happened to anybody else? Or is it just a fluke that nuked several hours of our progress? :P I'm hanging out in the instance hoping to hear back from a GM or somebody, but will accept the harsh reality imminently. At least we exited the arena undefeated, haha. (Clearly the boss fled fearing humiliation, etc.)