Three.js：在MeshFaceMaterial中使用cubeMaterials和不透明度-THREE.DoubleSide无法正常工作吗？

我正在使用ThreeJS r65创建一个简单的多维数据集。立方体的相对面具有相同的颜色分配：

var scene = new THREE.Scene(); 
var camera = new THREE.PerspectiveCamera(75, window.innerWidth/window.innerHeight, 0.1, 1000); 
var renderer = new THREE.WebGLRenderer(); 

renderer.setSize(window.innerWidth, window.innerHeight); 
renderer.setClearColor(0xffffff, 0); // bg color
document.body.appendChild(renderer.domElement); // displays canvas

camera.position.z = 13; // move away to see coord center
camera.position.y = 7;

controls = new THREE.OrbitControls(camera, renderer.domElement);

// CUBE
var geometry = new THREE.CubeGeometry(2,2,2);
var cubeMaterials = [ 
    new THREE.MeshBasicMaterial({color:0x33AA55, transparent:true, opacity:0.8, side: THREE.DoubleSide}),
    new THREE.MeshBasicMaterial({color:0x55CC00, transparent:true, opacity:0.8, side: THREE.DoubleSide}), 
    new THREE.MeshBasicMaterial({color:0x000000, transparent:true, opacity:0.2, side: THREE.DoubleSide}),
    new THREE.MeshBasicMaterial({color:0xFF0000, transparent:true, opacity:0.8, side: THREE.DoubleSide}), 
    new THREE.MeshBasicMaterial({color:0xFF0000, transparent:true, opacity:0.8, side: THREE.DoubleSide}), 
    new THREE.MeshBasicMaterial({color:0x5555AA, transparent:true, opacity:0.8, side: THREE.DoubleSide}), 
]; 
// Create a MeshFaceMaterial, which allows the cube to have different materials on each face 
var cubeMaterial = new THREE.MeshFaceMaterial(cubeMaterials); 
var cube = new THREE.Mesh(geometry, cubeMaterial);
scene.add( cube );

var render = function () { 
    requestAnimationFrame(render); 
    controls.update();
    renderer.render(scene, camera); 
};

render();

如果从顶部看向立方体，则会看到只有前两个绿色面在内部显示其颜色。其他4个面仅从外部显示颜色，而不从内部显示颜色。另请参阅图片：

难道我做错了什么？这是一个错误吗？还是由于WebGL？