I am guessing that the orientation of the filter doesn't really matter from the standpoint of effectiveness of filtration as both sides of it look to me about the same. That being said, it IS important that a used filter ALWAYS be replaced (if only inspected after removal) using the originally installed orientation so that the dirty side is never placed next to the cooling evaporator coils (which get cold and wet from condensed water and can easily get clogged from air borne dust, etc).
Looking at a recently dealer replaced filter (part# B7891-1FC0A -- FILTER ASSY-AIR), it is marked on the side with the tab:
"FRONT ^ A" (where ^ represents a solid arrow)
so it is somewhat ambiguous as to whether "A" means airflow or "FRONT" means front of the Leaf or front of the cooling coils.
I try to replace the filter myself (so that I can vacuum out that area which the dealership might not do), so I have interpreted the ^ as pointing towards the front of the Leaf (so that the pulling tab is on the top when installed). But my last Leaf inspection time I let the dealership replace it and I noticed later that the filter was replaced with the arrow pointing towards the rear of the car, i.e. towards the cooling coils which is, in fact, the direction of the airflow.
I might add that instructions seem to come with that filter, but they are in Japanese and the illustrations don't make the installed filter orientation clear.