I don’t see anything that sticks out in your code other than the names of the variables that your using, try
<?php
$related_posts = get_field('thanks_related_downloads', $pageid);
if( $posts ): ?>
<?php foreach( $related_posts as $related_post ): ?>
<?php echo get_the_title( $related_post->ID ); ?>
<?php endforeach; ?>
<?php endif; ?>
