Query fails to project subquery variables

With a database containing the quad in the data.nq file the following query:

SELECT ?uri_d WHERE {
VALUES ?type

{ <http://type1> }

GRAPH <http://graph1>

{ ?uri_d <http://p> ?type . }

}

return the expected result:

wrapping this query in another which projects the inner variable:

select ?uri_d where {
SELECT ?uri_d WHERE {
VALUES ?type

{ <http://type1> }

GRAPH <http://graph1>

{ ?uri_d <http://p> ?type . }

}
}

returns no matches. Changing the outer projection to * also returns no results.

However, projecting the unused variable ?type from the inner query does cause the result to be returned again:

SELECT ?uri_d WHERE {
SELECT ?type ?uri_d WHERE {
VALUES ?type

{ <http://type1> }

GRAPH <http://graph1>

{ ?uri_d <http://p> ?type . }

}
}

returns

again as expected.