Problem to add a css class in get_tag_more_links: only first element is classed

Hi,
I installed this interesting plugin and I made some improvements like adding a css class to “more tag link” to highlight current_tag.
I edited get_tag_more_links in this way but strangely it only highlights first tag element

public static function get_tag_more_links( $tags, $prefix = 'More posts: ' ) {
        global $wp_query;

        $tag_array = array();
        $output = '';

        if ($tags == '') {
            // if tags is empty then take from current posts
            if (is_single()) {
                $tag_array = wp_get_post_tags($wp_query->post->ID);
            }
        } else {
            $tag_array = explode(",", $tags);
        }

        if (count($tag_array) > 0) {

            # ADDED CODE
            //$current_tag = single_tag_title("", false);
            $current_tag =get_query_var('tag');

            //$output = '<p>' . $prefix;
            $output = '<ul class = "posts-by-tag-list">';

            foreach ($tag_array as $tag) {
                $tag_name = $tag;

                if (is_object($tag)) {
                    $tag_name = $tag->name;
                }

                $class = 'class="posts-by-tag-item"';
                if( $tag_name == $current_tag )
                    $class = 'class="posts-by-tag-item current_tag_item"';

                $output .= '<li ' .$class. '>';
                $output .= self::get_tag_more_link( $tag_name );
                $output .= '</li>';
            }

            //$output .= '</p>';
            $output .= '</ul>';
        }

        return $output;
    }

I have 2 tags and only first tag is highlighted when I click on it.
Why?

https://wordpress.org/plugins/posts-by-tag/