Hi,
Sorry for the issues you are having.
Unfortunately, there is not much I can do for 3rd party plugin conflicts.
However, I did try it out and found this coding to work:
<?php
$related_posts= get_field('relationship_field_name');
if( $related_posts): ?>
<?php endif; ?>
I did rename $posts to $related_posts from the example, but not sure if that is needed.
That is from ACF’s example on this page:
Relationship
The first example did not work for me, it returned the same results as you did and I think it is caused by the setup_postdata function.
That should get it working for you.
Let me know.
Thanks