This ^
Mobs* should run away as long as they are low HP, and if/when they regen enough HP back, they should immediately start chasing you again (unless fear/blind/etc), regardless of how far away they are or how long ago they have been aggroed initially. It's that simple.
Leashing on P99 is a like a big zit on the face of an otherwise pretty girl.
* who aren't flagged as non-fleeing such as some undead etc.
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