Our validator is not handeling this extension as XHTML but as a string.

The extension has valueString, so it should detect that based on the XHTML declaration inside the string

For more information see:
https://build.fhir.org/ig/HL7/fhir-extensions/StructureDefinition-rendering-xhtml.html

Extension.value[x] - x cannot be xhtml (https://hl7.org/fhir/datatypes.html#open)