我稍微修改了您的代码，并在Wordpress安装上进行了测试
如果使用过滤器（如“the\\u content”）接收作为参数的内容，则还必须返回此内容
这是带有注释的函数的工作版本。

 //this filter recieves $content, which you should always return, 
 //otherwise you will not see the content

 function posts_related($content){ 
    if (is_single()){

        $custom_query = new WP_Query( array( 
            \'posts_per_page\' => 8, 
            \'post__not_in\' => array(get_queried_object_id()), 
            \'orderby\' => \'rand\', 
        ));

        //lets define $related variable outside of while loop
        // and if statement to make it visible 
        $related = \'\';

    if ( $custom_query->have_posts() ) : 
        while ( $custom_query->have_posts() ) : $custom_query->the_post();

            //use functions with "get_" to recieve a value
            // and not echo it
            $permalink = get_the_permalink();
            $title = get_the_title();
        
            if ( has_post_thumbnail() ) { 

                //here we receiving just url with
                //get_the_post_thumbnail_url()
                $related .= \'<a href="\' . $permalink .  \'">
                                <img src="\' . get_the_post_thumbnail_url() . \'" />
                             </a>\';   
      
            }else{    
                $related .=  \'<a href="\' .  $permalink . \'"><b>\' . $title . \'</b></a>\';
            }
        endwhile;
        wp_reset_postdata();

        //return content + related posts
        //if is_singular and we have related post
        return $content . $related;

    else : 

        //if is_singular but no posts were found
        $related .= \'<p>Nothing to show.</p>\';

    endif;

    //if is_singular and we have related posts
    //return content + related posts 
    //or content + "nothing to show"
    return $content . $related;
    
}

//if page is not a single,
//just return content
return $content;

}