Migrating multiple values into entityreference field fails. Only first reference is imported.

The migrate field handler throws away multiple references during import. See code comment below.

Example Migration:

  // ... some migration code her
  $this->addFieldMapping('field_rel_lineart', 'PDM_lineArt_new')
     ->sourceMigration('AveryMediaItemMigration')
     ->separator(',')
   // ... more field mappings, etc.

  //Prepare Row some lines deeper.
  public function prepareRow($row) {
   
    $row->PDM_lineArt_new = '123,456,789';

  }

Migrate Handler Code:

  /**
   * Field-level prepare.
   *
   * Converts incoming data into the proper field arrays for entityreference
   * fields.
   *
   * @param object $entity
   *   The destination entity which will hold the field arrays.
   * @param array $field_info
   *   Metadata for the entityreference field being populated.
   * @param array $instance
   *   Metadata for this instance of the entityreference field being populated.
   * @param array $values
   *   Array of values for the entityreference field.
   *
   * @return array|null
   *   An array of entityreference fields.
   */
  public function prepare($entity, array $field_info, array $instance, array $values) {
    $arguments = (isset($values['arguments'])) ? $values['arguments'] : array();
    $language = $this->getFieldLanguage($entity, $field_info, $arguments);
    // Setup the standard Field API array for saving.
    $delta = 0;

    //@QUESTION: We always get an array, don't we? Why putting the array into another array with always one value?
    if (!is_array(reset($values))) {
      $values = array($values);
    }

    //@FIXME: This foreach loop runs always one time and therefore all other values are thrown away.
    foreach (array_filter($values) as $value) {
      $return[$language][$delta]['target_id'] = reset($value);
      $delta++;
    }
    // Return NULL for no values in the field.
    return isset($return) ? $return : NULL;
  }
}

Sending a patch to fix this later.