Default header not working correctly

The default header doesn’t show when a user wants to select the default header again. I have a multisite and I set a default header for all themes by setting it up in a mu-plugin file. I have:

$args = array(
	'default-image'          => content_url('/themes/s2/test.jpg'),
	'width'                  => 1000,
	'height'                 => 300,
//	'random-default'         => false,
//	'flex-height'            => true,
//	'flex-width'             => true,
//	'default-text-color'     => '',
//	'header-text'            => false,
//	'uploads'                => true,
//	'wp-head-callback'       => '',
//	'admin-head-callback'    => '',
//	'admin-preview-callback' => '',
);
add_theme_support( 'custom-header', $args );

This works fine – it displays a default header for all themes. In admin appearance>themes>header, users can change the default header with their own uploaded header or hide the header, by doing so, the default header moves to the ‘suggested’ area. Again, this is all good and users can upload any new header they like. But if a user wants to use the default header again (by clicking the suggested header), it doesn’t appear. It shows in the preview window, but as soon as you save it and refresh the page/visit the blog, it goes back to whatever the previous header was. How can I fix this?

I have tested it on a fresh install with the twenty ten theme and removed all plugins and all custom code (except custom header theme support).

Any help appreciated