MapPolygon not transparent fill with mapboxgl plugin

The MapPolygon is only filled with opaque colors if used with the mapboxgl plugin. I'd like to fill it with semi-transparent colors as is possible with the osm plugin (as well as fully transparent which also does not work, it's black instead). 

Did try the "mapboxgl.mapping.items.insert_before" plugin parameter as well as "mapboxgl.mapping.use_fbo" but no changes. 

        MapPolygon {
            id: thePolygon
            path: [
                { latitude: 60,   longitude: 10 },
                { latitude: 60.5, longitude: 10 },
                { latitude: 60.5, longitude: 11 },
            ]
            color: "#11223344"
            border.color: "blue"
            border.width: 2
        }