/**
 * Filter the available tags and convert all tag labels to lowercase.
 *
 * @param array $tags The available tags.
 * @return array The modified tags with lowercase labels.
 */
function my_wpf_lowercase_tags( $tags ) {
    // Loop through each tag and convert the label to lowercase.
    foreach ( $tags as $key => $label ) {
        $tags[ $key ] = strtolower( $label );
    }

    return $tags;
}
add_filter( 'wpf_get_setting_available_tags', 'my_wpf_lowercase_tags' );