There’s an odd glitch with the system that keeps you from exploiting mob movepaths. normally if you’ll try to snipe a creature from a spot they can’t reach the mob will become immune and be removed from combat. In some cases it gets set off under normal circumstances (i had it happen with several mobs including Vassar in the ascalonian catacombs).
It can be very frustrating to see a mob you’ve been fighting to inexplicably walk away as if you’re not there and force you to start over on it, and ive heard that it becomes problematic in areas with a high mob density. This is a bug and it needs to be addressed.