Just in case anyone else comes along searching for this, here’s what worked for me:
global $post;
$related_posts = get_field( 'related_posts' );
if( $related_posts ) :
foreach( $related_posts as $post ) : setup_postdata( $post );
// This worked for me
$output = apply_filters( 'the_content', $post->post_content );
echo $post->post_content;
endforeach;
wp_reset_postdata();
endif;