UV of AlphaMap and Map are shared in the three.js, how do I solve it?

Question

var tl = new THREE.TextureLoader();
var colorTex = tl.load( './env_half.png');
var alphaTex = tl.load( './glass_mask.png');

var planeGeometry = new THREE.PlaneBufferGeometry( 20, 10 );

colorTex.offset.setX(0.2); // Here's the problem

var planeMaterial = new THREE.MeshPhongMaterial( {
  side: THREE.DoubleSide,
  transparent: true,

  map: colorTex,
  alphaMap: alphaTex,
});

expected: When the colorTex moving, the mask can show different backgrounds.

actual: The colorTex and mask are moved together.

Thanks for your help! the three.js version is r142.