I only had enemies walk away if they hit the limit of the range from their initial location (which mostly happened when I was level 2 and needed to run away from level 4 opponents if support started arriving while I was attacking the closest of the group, or after I had defeated them but before I had time to heal). In those case, though, they were mostly invulnerable to attack until they got back to their regular spot (sometimes I could get a little damage in with my bow, but usually not).
My creature only walked back towards me if I moved too far away from it. Usually it wasn't that far, though, as I used a mage head (Magic Missiles) with my ranger. I tried a warrior head for awhile, and didn't notice it breaking off from the fight too much.
