A few times while fighting Rahyeh, The Black Sun in Empowered Monolith (300-400) the bird fails to come back after doing the fly-by void smoke attack during the last 25% of hp.
On one occasion I found the bird stuck down at the entrance to the fight, it was immobile and not attacking. On the most recent occasion it was nowhere and failed to return for quite some time, possibly over 5 mins. I had to die to exit the instance, which thankfully I was able to do with self-immolate.
It appears to be related to high dps, or possibly kill threshold triggering during an animation, but that could be purely coincidental.
I have had this happen 3 or 4 times now.