1: load the attached world (mirrored at http://www.mediafire.com/download/tekwe91j8zfeocb )
2: Fly under the platform. Watch the shadow disappear. This only happens with platforms at level 15, 31, 47, 63, etc,
CONFIRM FROM SCRATCH
1: start a flat world.
2: Build a large 1 block thick platform at y level 31 (or 15 or 63)
4: Load world again. fly under the platform. Shadow from the platform disappears as the lighting is updated based on the heightmap (which does not see the platform)