You're correct,
It is not the ground tile but the tile itself that controls whether or not path preview is drawn.
Since it is kinda hard for a player to determine if an air tile has been discovered I was trying to describe a quick check one can perform as to why this happens.
One that only works for this specific case (missing preview and fog of war visible on the lowest terrain with a 'floor').
In that case it is pretty safe to assume the preview is missing due to the most of the tiles in the column itself never passed visibility checks.
Sorry if my description caused confusion, i should have used better wording.