QXmlSchemaValidator fails to validate unique tag when path is an attribute

I've created an XML schema and document that validate correctly in XML Spy. But when attempting to validate them using a QXmlSchemaValidator it fails.

Build the attached example and press the Validate button to see the bug in action.

After the initial attempt fails, if you delete two of the three <at:author> lines in the document, leaving a single <at:author> and press Validate at that point the document will validate, but it should have validated with all three authors listed since all 3 authors had unique penNames which is what the <xs:unique> tag in the schema is checking for