acf-excerpt.php

<?php

/**
 * Set excerpt from ACF field
 */
add_action('acf/save_post', function($post_id) {

  $post_excerpt   = get_field( 'short_description', $post_id );

  if ( ( !empty( $post_id ) ) AND ( $post_excerpt ) ) {

    // Update post options array
    $update_post_excerpt_args = array(
      'ID'			=> $post_id,
      'post_excerpt'	=> $post_excerpt,
    );

    wp_update_post( $update_post_excerpt_args );

  }

}, 50);