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NPCs in a few Mastery Challenges behave erratically; jumping to their death, running into a fire, or leaving the available playing field/boundaries entirely.
I really like these challenges but it can get frustrating when you are so close to a Gold but have no way to complete said challenge because too many NPCs have committed Seppuku or a specific challenge requires all foes to be vanquished and one has, like Elvis, left the building.
(I do appreciate the ability to restart the challenge without backing completely out.)