I've encountered this bug as well, not 100% sure it's the same issue but I'm guessing it is.
From the testing I've done, the issue occurs when Vazruden the Herald (ID 17307) is killed, rather than the bosses, Vazruden and Nazan. This is most likely to occur from a different bug, in which, when Vazruden is engaged, Nazan is not automatically aggroed. If no one aggros Nazan before Vazruden dies, the fight will reset, and Vazruden the Herald will replace Nazan. At this point he can be killed, preventing the fight from being activated again but offering no loot. Alternatively, he can likely be killed in this form at the start, before he reaches the ground and splits into Vazruden and Nazan.
This would be most likely to occur when overlevel farming I'd assume, as you'd need enough DPS to kill Vazruden the Herald before he reactivates the encounter, though I imagine an at-level group could do it as well with enough DPS.
Hopefully this helps make it easier to fix.
