Similar bug just ended my previous group. The two dragons showed up for the fight, summoned some mobs, then just flew away. A little while later if we hadn’t killed them then the mobs they summoned would simply vanish. After a bit of waiting they would swoop in again and repeat, healing their health to full each time.
Might be related to another issue we experienced where the destiny’s edge npcs just chilled in the back of the humble; they didn’t even join us for the resetting dragons.
In the end my group just disbanded even though we were almost finished. Felt bad.