因为具体原因未知，因此无法提供具体解决方法。但是，可以尝试使用以下代码示例，该示例可将3D模型转换为2D Vector图像，从而解决转换问题。

// 定义视口参数
var viewport = new Autodesk.Viewing.Private.MyExtension.Viewport(canvas.width, canvas.height);
var vMatrix = new THREE.Matrix4();
var pMatrix = new THREE.Matrix4();
var aspectRatio = canvas.width / canvas.height;

// 使用3D相机和投影矩阵进行设置
var camera = new THREE.PerspectiveCamera(45, aspectRatio, 0.1, 10000);
var cameraEye = viewer.navigation.getEyePosition();
var cameraTarget = viewer.navigation.getTarget();
var upVector = viewer.navigation.getCameraUpVector();
camera.position.set(cameraEye.x, cameraEye.y, cameraEye.z);
camera.up.set(upVector.x, upVector.y, upVector.z);
camera.lookAt(new THREE.Vector3(cameraTarget.x, cameraTarget.y, cameraTarget.z));
vMatrix = camera.matrixWorldInverse;
pMatrix = camera.projectionMatrix;

// 使用Three.js库的WebGLRenderer方法
var renderer = new THREE.WebGLRenderer({
  alpha: true,
  canvas: document.getElementById('canvas')
});

// 定义一个渲染器，最大程度保持原始颜色
var gl = renderer.getContext();
gl.disable(gl.DEPTH_TEST);
gl.disable(gl.CULL_FACE);
gl.disable(gl.BLEND);
renderer.setClearColor(0xffffff, 0);
renderer.gammaInput = true;
renderer.gammaOutput = true;

// 获取要转换的模型
var model = NOP_VIEWER.model;

// 定义材质
var material = new THREE.MeshPhongMaterial({
  color: 0x088da5,
  shading: THREE.SmoothShading
});

// 渲染方法
function renderMesh(mesh) {
  mesh.traverse(function(child) {
    if (child instanceof THREE.Mesh) {
      child.material = material;
    }
  });

  // 创建一个新的场景
  var scene = new THREE.Scene();
  scene.add(mesh);

  // 渲染新场景
  renderer.render(scene, camera);

  // 生成PNG图像
  var imageData = renderer.domElement.toDataURL('image/png');
  console.log(imageData);

  // 将PNG图像添加到页面
  var img = new Image();
  img.src = imageData;
  document.body.appendChild(img);
}

// 遍历模型实例
var it = model.getData().instanceTree;
it.enumNodeFragments