Unity AvatarMask.AddTransformPath() doesn't work

Question

Recently , I was trying to figure out how to make animations played in Unity's editor mode.

I tried to play animations by using PlayableAPI and "ExecuteAlways" attribute(make sure the script will runs correctly in editor mode).And when I was attempting to using "AvatarMask" for Layering animations,I found this API didn't work.

codes and Unity scene are below

public void Play()
    {
        IKSwitch = true;
        _anim.enabled = true;
        SceneView.FocusWindowIfItsOpen<SceneView>();
        AnimationLayerMixerPlayable mixer;
        try
        {
            mixer = AnimationLayerMixerPlayable.Create(_graph, 2);
        }
        catch (Exception e)
        {
            Debug.Log("Initialize failed, re-initializing.....");
            InitGraphAndOutput();
            return;
        }
        //Create Avatar Mask
        AvatarMask mask = new AvatarMask();
        if (handTransform)
        {
            mask.AddTransformPath(handTransform, true);
        }


        //Connect AnimationClipPlayable
        mixer.ConnectInput(0, AnimationClipPlayable.Create(_graph, clip), 0, 1);
        AnimationClipPlayable empty = AnimationClipPlayable.Create(_graph, clip2);
        empty.SetApplyPlayableIK(true);
        mixer.ConnectInput(1, empty, 0,1);

        mixer.SetLayerMaskFromAvatarMask(1, mask);

        //Connect layerMix to output
        output.SetSourcePlayable<AnimationPlayableOutput, Playable>(mixer, 0);
        _graph.Play();
    }

And the appearance of the graph is playing clip2 instead of playing mixed animation as I presumed.

I was expecting that clip2 would only take effect on specified parts of character bones