Virtual containers media children refer to their content through the @id property.
They should use "reference items" and refer to the content through the @refID property.

This seems not to be mandatory as written in the UPnP av ContentDirectory service, but it is recommended:
"When the ContentDirectory service implementation contains multiple objects that refer to the same content, it is RECOMMENDED that the implementation use reference items for all but one of those objects."

Also the Tracker plugin works as described above.