Export does not always specify xsd:datetime for 'dumbed down' version of EDTF string

In my Wikibase the valid EDTF string XXXX maps correctly maps to +0000-00-00T00:00:00Z in xsd:datetime, but without the proper datatype in the export to Blazegraph. So "+0000-00-00T00:00:00Z" should be "+0000-00-00T00:00:00Z"^^xsd:dateTime

So this TTL should have an additional ^^xsd:dateTime

wd:Q483 a wikibase:Item ;
	wdt:P19 "XXXX"^^xsd:edtf,
		"+0000-00-00T00:00:00Z" ;
	wdt:P21 "1799"^^xsd:edtf,
		"1799-01-01T00:00:00Z"^^xsd:dateTime ;

This query:

SELECT DISTINCT ?date ?type
WHERE {
  BIND(wd:Q483 AS ?item)
  ?item wdt:P19 ?date.
  BIND(datatype(?date) AS ?type)
} 

should not yield

Start by reproducing the Q483 export and the supplied Blazegraph query, then trace where the EDTF value XXXX is converted into the +0000-00-00T00:00:00Z literal. Done means the converted literal is exported with the xsd:dateTime datatype rather than xsd:string.