This is something new in patch 5, and it was implemented to stop an exploit where if you used Darkness, or Fog Cloud on a vendor they would stay in it and let you steal everything from them without penalty. So now NPC's will try to run out of it which makes sense.
But you are saying used in combat they won't engage you in melee in the Darkness spell, or they run out of it? Honestly that sort of makes sense. Although yeah, there should be a chance of falling off a cliff blind. Wonder if that would be hard to implement.
