This adds ACF content to the posts content if it is called by the feed.

custom-rss-wordpress-feed

// This adds ACF content to the posts content if it is called by the feed.
// SRC: https://digwp.com/2012/10/customizing-wordpress-feeds/ AND http://www.paulund.co.uk/add-custom-fields-to-rss-feed
function fields_in_feed( $content ) {  

  if( is_feed() ) {

    $post_id = get_the_ID();  
    $acf = get_post_meta( $post_id, 'newsletter-posts-specific', true ); //using 'true' here is vital
    $output = '';
    
    if ( $acf ) { 

      $output .= '<div class="newsletter-posts-specific">';
        
      $output .= '<br /><h2>In this edition</h2><br />';
       
        // Loop through the ACF relationship field
        foreach ( $acf as $acf_post_id ) { 

          $excerpt = apply_filters( 'the_excerpt', get_post_field( 'post_excerpt', $acf_post_id ) ); 
          $image = get_the_post_thumbnail( $acf_post_id, 'large' );

            $output .= $image;
            $output .= '<br /><h3>' . get_the_title( $acf_post_id ) . '</h3>';
            $output .= $excerpt;
            $output .= '<a href="' . get_permalink( $acf_post_id ) . '">read more...</a><br /><br />';

        } // End ACF loop
      
      $output .= '</div>';
     
    } // End if ACF

    $content = $content.$output;  
  
  } // End if feed

  return $content;  

} // End function

add_filter('the_content','fields_in_feed');