SPARQL module mixes up datatypes and language tags of literals?

(... and drops results that are present in virtuoso).

I'm querying a data set that includes some XML literals, i.e. literals with a datatype (http://www.w3.org/1999/02/22-rdf-syntax-ns#XMLLiteral). Furthermore, some terms in the original RDF/XML have both a datatype and a language tag.

<skos:Concept rdf:about="&msc2010;19-XX">
        <skos:prefLabel xml:lang="en" rdf:parseType="Literal"><mml:math alttext="$K$"><mml:mi>K</mml:mi></mml:math>-theory </skos:prefLabel>\
[...]

When I submit queries that filter by language type, the results obtained with sparql.module are confused, resulting in some empty fields (see attached screenshot). On the other hand, in Virtuoso's sparql interface, the same query yields results like this:

http://msc2010.org/resources/MSC/2010/16-XX     "Associative rings and algebras"@en
http://msc2010.org/resources/MSC/2010/17-XX     "Nonassociative rings and algebras"@en
http://msc2010.org/resources/MSC/2010/18-XX     "Category theory; homological algebra"@en
http://msc2010.org/resources/MSC/2010/19-XX     <n0:math xmlns:n0="http://www.w3.org/1998/Math/MathML" alttext="$K$"><n0:mi>K</n0:mi></n0:math>-theory
http://msc2010.org/resources/MSC/2010/20-XX     "Group theory and generalizations"@en
http://msc2010.org/resources/MSC/2010/22-XX     "Topological groups, Lie groups"@en

You can see that the @en tag is missing in the returned result, but content is there!

If you would like to investigate this further, you can access our Virtuoso UI at http://li311-58.members.linode.com:8890/sparql and a simple Drupal front-end that can interface with this endpoint at http://li311-58.members.linode.com/squery -- the query I used is:

PREFIX msc:<http://msc2010.org/resources/MSC/2010/>
PREFIX skos:<http://www.w3.org/2004/02/skos/core#>
select distinct ?child ?label where {
?child skos:topConceptOf <http://msc2010.org/resources/MSC/2010/MSC2010> .
?child skos:prefLabel ?label .
FILTER langMatches( lang(?label), "en" )  }
order by ?child