I don't have classic evidence for this (sorry), but hopefully y'all already understand how this mechanic should work, and this is just an implementation issue.
As I remember things on live, a fleeing mob would
always come back to you, as soon as their HP regenerated enough that they stopped fleeing. I do not believe it was possible in classic EQ for a mob to flee from low HP, flee far enough away that they "unleash" (lose agro), and then start healing up (to full) without returning to their spawn point.
However, exactly that happened to me. I was on the south wall of Kithicor, about -900, 4000, and I beat
C`Habrel Sharlok down to the point of fleeing. He then wandered off into the forest somewhere ... and promptly forgot all about me and healed up, without returning to his spawn point.
Much later he finally returned to his spawn point ... but again, I don't recall any scenario like this happening on live.
I think the fix is just to limit mobs so they don't wander outside their "unleash" range. And if this is set a per zone/mob basis, Kithicor Sharloks need correcting (or at least C`Habrel does).