I faced the same problem and, as a good and nice plugin, BBQ offers a helpful filter :)
The following filter only fires in admin side and when doing Ajax, which is the moment where BBQ's union key gets confused about Jetpack's grunion call:
add_filter( 'query_string_items', 'b5f_filter_bbq_for_jetpack' );
function b5f_filter_bbq_for_jetpack( $items )
{
if( is_admin() && defined('DOING_AJAX') && DOING_AJAX )
{
$key = array_search( 'union', $items );
unset( $items[ $key ] );
}
return $items;
}