Virtuoso Bug: Query incorrectly evaluates to empty result set; breaks keyword search over literal facet values

[Aklakan]

The following query incorrectly returns 0 results on the Virtuoso deployed at http://cord19.aksw.org/sparql

The actual query has a VALUES clause as the first UNION member (instead of the empty graph pattern) but it suffers from the same empty result set problem

SELECT  *
WHERE
  { {
     SELECT  ?o
        WHERE
          {
              {} # Empty for the sake of a minimal example
            UNION
              { ?s  ?p  ?o FILTER ( ?p = <http://id.loc.gov/ontologies/bibframe/title> )}
          }
      }
      OPTIONAL { ?o  <http://www.w3.org/2000/01/rdf-schema#label>  ?v_1 }

    FILTER ( regex(str(?o), "met", "i")  )
  }

LIMIT 100

• Interestingly the query works when filtering?p to <http://example.org/dbpediaSubject> (instead of <http://id.loc.gov/ontologies/bibframe/title>).
  The titles are all literals and the dbpediaSubjects are all IRIs - so apparently the optional block fails to evaluate for literals.

• The query APPEARS to work but actually DOESN'T when omitting the str function and writing directly regex(?o, ...):
  It seems as if the regex then evaluates to true regardless of ?o's value.

There may be some wacky optimization in place regarding variables in subject positions that are bound to literal values...