由于a bug in Wordpress core.

Wordpress ticket中提供了一个解决方法

function disable_canonical_redirection_for_tag_feeds() {
if ( is_feed() && is_tag() )
    remove_action( \'template_redirect\', \'redirect_canonical\' );
}
add_action( \'wp\', \'disable_canonical_redirection_for_tag_feeds\' );