Can't draw polygon on ground level with 3D tiles

I’m trying to draw a polygon on top of a 3D tile.

This is very close, the only issue is that no matter what I do I can’t have a thin polygon on ground level.

If I make it thin without setting heightReferences it just hovers

If I do set them, i can’t get the thin layer I want.
This is the code I’m using:

this.viewer.entities.add({
      name: 'Polygon',
      polygon: {
        hierarchy: new PolygonHierarchy(coord),
        material: Color.BLUE.withAlpha(0.5),
        outline: true,
        outlineColor: Color.BLACK,
        heightReference: HeightReference.RELATIVE_TO_GROUND,
        extrudedHeightReference: HeightReference.CLAMP_TO_GROUND,
        extrudedHeight: 0.001
      },
      position: new Cartesian3(viewport.getNorthEast().lng(), viewport.getNorthEast().lat(), -20)

    });

3D tileset is from Google Maps API.

For reference I’d really need something at least half this height:

and ideally the ability to set arbitrary heights respecting the 3D tile structure heights.