Not Sure Why 'post__not_in' Is Not Working – Nested Loops

Hi Everyone,

I am very new to WordPress Development. I am in mind 4th day so far, so I still don’t know a lot. I wonder if you can help me. I am editing single.php theme template. I want to add Related Posts section that references links to blog articles that have tags similar to one of the current blog article. The output would look like this:

Current Article Name
Some content of the current Article Name

Related Posts:
One Related Article
Two Related Article

Current Article Name should NOT be referenced under Related Posts, yet it does. Here is the code of single.php:

<main id="main" class="site-main" role="main">
			<?php if (have_posts()):?>
            	<?php while(have_posts()) : the_post(); ?>
                	<?php the_title('<h1>','</h1>',true); ?>
                    <p class="auth">Written by <?php the_author_posts_link(); ?> on <?php the_time('F j, Y'); ?></p>
                    <p>Found in: <?php the_category(', '); ?></p>
                    <p><?php the_content(); ?></p>
                    <?php the_tags('<p>Tags: ',' / ','</p>'); ?>
                    <?php
                    //get an array of tags associated with this post
					$tags = wp_get_post_terms(get_the_ID());
					?>
                    <pre>
                    	<?php print_r($tags); ?>
                    </pre>
                    <?php
					if ($tags)
					{
						echo '<h2>Related Terms:</h2>';
						foreach ($tags as $tag)
						{
							$firstTag = $tag->term_id;
							$args = array(
							'tag__in'=>array($firstTag),
							'post__not_in'=>array($post->ID),//it has $post->ID in the book. I used a different way to retrieve current post id
							'posts_per_page'=>5,
							'ignore_sticky_posts'=>1
							);
							//create nested custom Loop
							$relatedPosts = new WP_Query($args);
							if ($relatedPosts->have_posts())
							{
								while($relatedPosts->have_posts())
								{
									$relatedPosts->the_post();
							?>
								<p><a href="<?php the_permalink(); ?>"><?php the_title(); ?></a></p>
							<?php
								}
							}
						}
					}
					?>
                <?php endwhile; ?>
            <?php endif; ?>
		</main><!-- .site-main -->