Enemy units start to idle if they are out of line of sight. Just teleport someone from high ground to a far away location. He will simply stop moving and ignore the fight till it ends. Also if you start shooting at someone out of line of sight enemy will not fight back sometimes and just die without combat ever starting. This is not 100% trigger happens here and there tbh.
This bug is the same as the bug described in the following posts and I think are the same
http://larian.com/forums/ubbthreads.php?ubb=showflat&Number=651729#Post651729http://larian.com/forums/ubbthreads.php?ubb=showflat&Number=651691#Post651691