method chaining not working

The Field Mappings doc page (https://www.drupal.org/node/1133448) says that methods can be chained, and I've done this many times myself. However, the separator works here:

    // Entity references
    $this->addFieldMapping('field_rights_holder', 'Rights Holder')
      ->separator('|');

But does not work here:

    // Entity references
    $this->addFieldMapping('field_rights_holder', 'Rights Holder')
      ->separator('|')
      ->sourceMigration(array('WorkNode')); 

In the latter case, source values simply are not separated. In the former they are. Is this a bug?