Hi @mikosworld,
Thanks for the post.
The issue could be a result of the multiple calls to the setup_postdata() function.
You can create a new loop without this function like so:
<?php
$posts = get_field('relationship_field_name');
if( $posts ): ?>
<ul>
<?php foreach( $posts as $p ): // variable must NOT be called $post (IMPORTANT) ?>
<li>
<a href="<?php echo get_permalink( $p->ID ); ?>"><?php echo get_the_title( $p->ID ); ?></a>
<span>Custom field from $post: <?php the_field('author', $p->ID); ?></span>
</li>
<?php endforeach; ?>
</ul>
<?php endif; ?>
