Ninja 65 quest bugged?

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Date & Time: 12/15/2018
Frequency: Once, but I'm not sitting through a twenty minute mission for it to happen again
World name: Lamia
Monster name: Zakuro Brightblood
Class/Level: Ninja 65
Party or solo: Solo
Area and coordinates: The Ruby Sea x35, y21
Steps:
1. Tried to do the Ninja job quest for this level
2. After wearing the boss down to <10% health, the game said I failed the Duty and kicked me out
3. ?????

The duty will fail if important NPCs die, so make sure that doesn't happen.

Go slow.

The problem is is if you move to quick the npcs can't keep up. So the when you attack one of the guards everyone becomes hidden and if the npcs haven't caught up. They'll unhide right in the middle of that big group you have to stealth through and get absolutely hammered.

Basically don't engage the guards till the npcs have caught up with you