Have you tried ImageDescriptor attribute?

  public class ImageData : MediaData
  {
    /// <summary>
    /// Gets or sets the generated thumbnail for this media.
    /// </summary>
    [ImageDescriptor(Width = 96, Height = 96)]
    public override Blob Thumbnail
      {
        get { return base.Thumbnail; }
        set { base.Thumbnail = value; }
      }
  }

Try the Clear Thumbnail Properties job

Yes I did both of these. However thumbnails are not regenerated when I run the job, when I look in the Azure storage explorer I still see the existing files.