Not 100% sure but, being the get_field() is a global function you MIGHT have to specifically target the post your are getting the related fields from…
Maybe doing something like the following would work
global $post;
$post_id = $post->ID;
$posts = get_field( "related_posts", $post->ID );
I’m assuming your above code displays the relationship data properly. Please post the code that is NOT working.