感谢@NikolaIvanovNokolov为解决方案提供了正确的路径！

// Add action hook to link genre to movie selection
add_action('save_post', 'save_genre');

// Action hook to link Genre taxonomy to the post
function save_genre($post_id) {
    // Remove reference to any old genres
    wp_set_post_terms($post_id, NULL, 'genre');

    // Add genre reference
    $movie_ids = get_the_terms($post_id, 'movie');
    if ($movie_ids && !is_wp_error($movie_ids)) {
        foreach ($movie_ids as $movie_id) {
            $genre_slug = get_tax_meta($movie_id, 'movie_tags_genre');
            if ($genre_slug) {
                $genre = get_term_by('slug', $genre_slug, 'genre');
                wp_set_post_terms($post_id, $genre->term_id, 'genre');
            }
        }
    }
}