WordPress Related Post by tags in Single.php

The code shown isn’t working as wished, what is wrong?

you’re using only the first tag : $first_tag = $tags[0]->term_id;
to grab posts from all tags you have to iterate the whole array until a max of 5. The code below ( untested) grabs a max of 25 posts theoretically using up to 5 tags..BUT if you have 25 posts belonging to the first tag you will have only posts related to the first tag anyway:

$tags = wp_get_post_tags($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="https://wordpress.stackexchange.com/questions/356552/<?php the_permalink() ?>" rel="bookmark" title="Permanent Link to <?php the_title_attribute(); ?>"><?php the_title(); ?></a>
 <?php
  endwhile;
  }
wp_reset_query();
}

to be sure to grab 5 posts from 5 tags you should run 5 queries ( unless there’s a more efficient way to do that of which I’m not aware of )

$tags = wp_get_post_tags($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="https://wordpress.stackexchange.com/questions/356552/<?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();
}