显示的代码没有按预期工作，怎么了？

您只使用了第一个标记：$first_tag = $tags[0]->term_id;要从所有标签中获取帖子，您必须迭代整个数组，直到最大值为5。下面的代码（未测试）理论上最多可以使用5个标签获取25篇文章。。但是，如果您有25篇属于第一个标记的帖子，那么无论如何，您只会有与第一个标记相关的帖子：

$tags = wp_get_post_tags($post->ID);
if ($tags) {
  echo \'Related Posts\';
  $max=5;
  $arrayTags=[];
  for($k=0;$k<$max;$k++){
    $arrayTags[]=$tags[0]->term_id;
  }

  $args=array(
    \'tag__in\' => $arrayTags,
    \'post__not_in\' => array($post->ID),
    \'posts_per_page\'=>25,
    \'ignore_sticky_posts\'=>1
  );
  $my_query = new WP_Query($args);

  if( $my_query->have_posts() ) {
    while ($my_query->have_posts()) : $my_query->the_post(); ?>
     <a href="<?php the_permalink() ?>" rel="bookmark" title="Permanent Link to <?php the_title_attribute(); ?>"><?php the_title(); ?></a>
 <?php
  endwhile;
  }
wp_reset_query();
}

$tags = wp_get_post_tags($post->ID);
if ($tags) {
    echo \'Related Posts\';
    $max=count($tags) >5 ? 5 : count($tags) ;
    $idsToExclude=[$post->ID];
    //ob_start();
    for($k=0;$k< $max ;$k++){
        $args=array(
        \'tag__in\' => $tags[$k]->term_id,
        \'post__not_in\' => $idsToExclude,
        \'posts_per_page\'=>5,
        \'ignore_sticky_posts\'=>1 // caller_get_posts is deprecated
        );
        $my_query = new WP_Query($args);
        if( $my_query->have_posts() ) {
            while ($my_query->have_posts()) : $my_query->the_post(); 
            if(!in_array(get_the_id(),$idsToExclude)){
                $idsToExclude[]=get_the_id();   // add this id to the exclusion for the next query
            }           
            ?>
            <a href="<?php the_permalink() ?>" rel="bookmark" title="Permanent Link to <?php the_title_attribute(); ?>"><?php the_title(); ?></a><br>
            <?php
            endwhile;
        }

    }
    //echo ob_get_clean();
    wp_reset_query();
}