When you order a unit to face a direction and order them to face another direction, they will follow a "face the enemy" command, instead of facing the desired direction.
So, the only way to change to from a "custom" direction to another is by giving the "face this direction" order 2 times...
So, the only way to change to from a "custom" direction to another is by giving the "face this direction" order 2 times...