I’ve finally figured it out. This is what I used to be able to show my related posts on WooCommerce product pages:
<?php
$vendorposts = get_field('related_posts');
if( $vendorposts ): ?>
<div class="latest_post_holder boxes three_columns two_rows">
<ul>
<?php foreach( $vendorposts as $post): // variable must be called $post (IMPORTANT) ?>
<?php setup_postdata($post); ?>
<li>
<?php if ( has_post_thumbnail( $rp->ID ) ) : ?>
<?php echo get_the_post_thumbnail( $rp->ID, 'regular-posts' ); ?>
<?php else : ?>
<img src="<?php echo get_template_directory_uri(); ?>/img/no-image.png" alt="" />
<?php endif; ?>
<a href="<?php the_permalink(); ?>"><?php the_title(); ?></a>
</li>
<?php endforeach; ?>
</ul>
</div>
<?php wp_reset_postdata(); // IMPORTANT - reset the $post object so the rest of the page works correctly ?>
<?php endif; ?>
