An export command that includes rdfs:isDefinedBy in the header doesn't export any values for this column.
I believe the issue is in ExportOperation.getPropertyValues() where it attempts to find an entity in the ontology to use as the value for this column. Since the target for rdfs:isDefinedBy is usually an IRI for an ontology (not a term in an ontology), the call ontology.getEntitiesInSignature(iri) never returns an entity (the IRI for the ontology and any imported ontologies are not in the signature for the ontology ) so no value is created.
This could be handled either by checking for the IRIs for the ontology and imported ontologies in addition to the term IRIs or just using the IRI as the value if no entities are found for that IRI.