ThreeJS GLTFExporter doesn't export PBR material maps

I have a simple Cube with a PBR Material. Used maps are color, metalness, roughness, bump. When I export the scene via this code, the exported glb is missing all maps but the color. Is this a bug in ThreeJS?

e = new THREE.GLTFExporter();

e.parse(STAGE.scene.mesh, (glb) => {
   let blob = new Blob([glb], { type: "application/octet-stream" });
   let d = document.createElement('a');
       d.href = window.URL.createObjectURL(blob);
       d.download = "orbis.glb"
       document.body.appendChild(d);
       d.click();
       document.body.removeChild(d);
}, {binary: true});

You should use different textures. The reason for this is the gltf specification:

• gltf uses a normalMap instead of a bumpMap, so bumpMaps are not exported
• metalnessMap and roughnessMap are only exported if they are the same texture because gltf uses a single texture for both (see https://github.com/KhronosGroup/glTF/tree/master/specification/2.0#metallic-roughness-material and https://github.com/mrdoob/three.js/issues/14940 for a feature request to merge those textures in the exporter)