ambientocclusion in custom blockmodels is not used

In my (unfinished) blockmodel with the ambientocclusion tag, there is no effect from the change:

dispenser_vertical.json

{
    "ambientocclusion": false,
	"textures": {
        "all": "blocks/bautisch"
    },
    "elements": [
        {   "from": [ 0, 11, 0 ],
            "to": [ 16, 15, 16 ],
            "faces": {
                "down":  { "texture": "#all"},
                "up":    { "texture": "#all"},
                "north": { "texture": "#all"},
                "south": { "texture": "#all"},
                "west":  { "texture": "#all"},
                "east":  { "texture": "#all"}
            }
        }
    ]
}