I think you may be using the wrong code to get and display the gallery. See the “Create WP Gallery” section of this page http://www.advancedcustomfields.com/resources/gallery/
In your case you should probably be using something like:
$image_ids = get_sub_field('movie_stills', false, false);
$shortcode = '[gallery ids="' . implode(',', $image_ids) . '"]'
echo do_shortcode( $shortcode );