I’m using a script (cyb-glossary) to sort the posts alphabetically according to the first letter of the title. The script is very simple and it works. First register a taxonomy as a hidden tag, create the tag and assign it to each post. Then list the letters of the alphabet as a link for each tag. The problem that he has and that I do not understand why, is that when I click on each letter/tag it directs me to a single post and not to the tag.php template where all the posts that start with the letter of the clicked tag are listed

add_action('save_post','title_set_first_letter');
function title_set_first_letter( $post_id ){
 
    // skip autosave
    if( defined('DOING_AUTOSAVE') && DOING_AUTOSAVE ) {
         return;
    }
 
    // limit to term post type
    if( isset($_POST['post_type']) && 'post' != $_POST['post_type'] ) {
         return;
    }
    
     // Check permissions
    if( !current_user_can('edit_post', $post_id ) ) {
        return;
    }
 
    //Assign first letter of the title as titulos term
    if( isset($_POST['post_title']) ) {
        //TODO: skip no-sense words, characters, etc
        $$alphabet_term = mb_strtolower( mb_substr( $_POST['post_title'], 0, 1 ) );
        wp_set_post_terms(
        $post_id,
        $alphabet_term,
        'titles'
        );
    }
 
    //delete the transient that is storing the alphabet letters
    delete_transient('archive_alphabet');
 
}

// Set ascendent order by name in "terms" post type and titulos taxomony archives
add_action( 'pre_get_posts', function( $query ) {

  if( ! $query->is_admin && $query->is_main_query() && ( $query->is_tax( 'titles' ) || $query->is_post_type_archive( 'post' ) ) ) {
    $query->set( 'order', 'ASC' );
    $query->set( 'orderby', 'title' );
  }
  
} );



function title_the_alphabet_menu() {
     $taxonomy ='titles';
     // save the terms that have posts in an array as a transient
     if( false === ( $alphabet = get_transient('archive_alphabet') ) ) {
          // It wasn't there, so regenerate the data and save the transient
          $terms = get_terms($taxonomy);
          $alphabet = array();
          if($terms){
              foreach($terms as $term){
                  $alphabet[]= $term->slug;
              }
          }
          set_transient('archive_alphabet', $alphabet );
     }?>
     <ul id="alphabet-menu">
         <?php
         foreach(range('a','z') as $i) {
 
             $current = ($i == get_query_var($taxonomy)) ? "current-menu-item" : "menu-item";
             if(in_array( $i, $alphabet )) {
                 printf('<li class="az-char %s"><a href="%s">%s</a></li>', $current, get_term_link( $i, $taxonomy ), $i );
             } else {
                 printf('<li class="az-char %s">%s</li>', $current, $i );
             }
 
          }
          ?>
     </ul>
<?php
}

The link appears fine but it redirects me to a single post, why?? <li class="az-char menu-item"><a href="http://localhost/library/titles/a/">a</a></li>