The Patient Demographics Query for Mobile (PDQm) Profile defines a lightweight RESTful interface to a patient demographics supplier leveraging technologies readily available to mobile applications and lightweight browser based applications.
It is should. We have never forbidden non compliant resources to be returned. In MHD I added a warning to clients that they need to be robust to non compliant resources. This is simply part of Postel's Law.
From https://build.fhir.org/ig/IHE/ITI.PDQm/branches/main/ITI-78.html#23784221-patient-resource-definition-in-the-context-of-query-patient-resource-response
"The Patient Resource(s) contained within the Query Patient Resource Response message should conform to Patient Profile for PDQm."
Should this "should" be a "shall"?