Bug fixes for Engine Nine 1.6 as suggested by yufeih.

MaterialPaintGroup.cs

/// Sets the mask textures of the material group.
/// </summary>
public static void SetMaskTextures(MaterialGroup materialGroup, System.Collections.IList value)
{
    if (value == null)
        throw new ArgumentNullException("value");

    AttachablePropertyServices.SetProperty(materialGroup, MaskTexturesProperty, value);

    MaterialPartCollection materialParts = materialGroup.MaterialParts as MaterialPartCollection;
    if (materialGroup != null && materialParts != null)
        materialParts.Bind(materialGroup);
}

/// <summary>
/// Sets the mask texture scale of the material group.
/// </summary>
public static void SetMaskTextureScale(MaterialGroup materialGroup, Vector2 value)
{
    AttachablePropertyServices.SetProperty(materialGroup, MaskTextureScaleProperty, value);

    MaterialPartCollection materialParts = materialGroup.MaterialParts as MaterialPartCollection;
    if (materialGroup != null && materialParts != null)
        materialParts.Bind(materialGroup);
}

It works!