ACF custom fields: Auto populate end date with the start date value, when it is empty.

functions.php

<?php 
// Auto-populate end date if it is empty.
function mafs_update_data_final( $value, $post_id, $field ) {
  
  //NOTE: don't use get_field() because it retrieves the value
  //in a preformatted way different as it is saved in database
  $data_final = get_post_meta( $post_id, 'doc_data_final', true );
  $data_inicial = get_post_meta( $post_id, 'doc_data_inici', true );

  //Check if data_final is empty (when saving) and data_inicial
  //has a value registered, then assign it to the empty one.
  if ($data_final == '' && $data_inicial != '') {
    $value = $data_inicial;
  }
  
  return $value;
  
}
// Write the name of the affected custom field in the filter
// in our case: doc_data_final
add_filter('acf/update_value/name=doc_data_final', 'mafs_update_data_final', 10, 3);