"cullface": "down" does not always hide faces.

json command:
"cullface": "down"
does not hide the face it should affect, when the model itself is not a solid block.

So if you place a Grass block on top of another Grass block it works. But if u place a nonsolid block like Brick Stairs on top of a Grass block, the faces flagged as cullface:down are not hidden on the Stairs.