Keyword marked as person by hand does not export the <Iptc4xmpExt:PersonInImage> metadata tag

If a person is detected by face recognition, the exported image will have the metadata:
<Iptc4xmpExt:PersonInImage>
  <rdf:Bag>
    <rdf:li>John Doe</rdf:li>
  </rdf:Bag>
</Iptc4xmpExt:PersonInImage>

The keyword will be marked as "Person".

But when you mark that by hand (by clicking the checkbox "Person" manually in a keyword), the above metadata will not be exported.