Im not sure wether this has been discussed, Im too lazy to check. :-) sorry!
first off - description. when you take the main quest for the shadow thieves (the one where you are asked to steal the components for a Halruuan skyship) you can also do quests for Narlen Darkwalk, resulting in an encounter where he helps you (due to spoilers I will not say any more. :-))
Ive been experimenting a bit and Ive found out, that this bug is connected to Rasaad - if you dont recruit him right away and get attacked by Nimbul, he helps you and once Nimbul is dead, Rasaad enters conversation with you for some reason. afterwards the abovementioned bug will spawn.
if you recruit Rasaad before meeting Nimbul, the bug doesnt spawn.
the bug itself is somewhat useful, since you gain more exp due to it, buuuuut you know ... :-)