A blending issue perhaps? According to some posts I read, the order in which tiles are declared is very important as ones declared higher up take priority over those declared lower down. By priority I am referring to blend priority, how the tiles interact. It is possible that the way the "Abyss (Outland)" texture works is not compatible unless declared a certain priority. Especially since it is meant to mix with the cliff type.
Also possible. WC3 is riddled with bugs.