I can not reproduce this issue, either by trying your example query in the workbench or by programmatically executing it and serializing the result to RDF/XML.

Can you give me some more details about the exact circumstances in which this problem occurs?

Jeen,

I have tested again vusing several ways using HTTP POST

a) curl
curl -X POST -d @DatasetQuery.txt http://www.proxml.be:8080/openrdf-sesame/repositories/OWMS

within the response
_:node130 dcterms:format "application/rdf xml" .

b) HTTP client on Mac
       <rdf:Description rdf:nodeID="node122">
<dcterms:format>application/rdf xml</dcterms:format>
</rdf:Description>

c) Firefox REST client
<rdf:Description rdf:nodeID="node113">
<dcterms:format>text/rdf n3</dcterms:format>
</rdf:Description>

The content of DatasetQuery.txt is
query=PREFIX overheidbuza:<http://standaarden.overheid.nl/buza/terms/>
PREFIX skosxl:<http://www.w3.org/2008/05/skos-xl#>
PREFIX foaf:<http://xmlns.com/foaf/0.1/>
PREFIX rijksoverheid:<http://standaarden.overheid.nl/rijksoverheid/terms/>
PREFIX inspectieloket:<http://standaarden.overheid.nl/inspectieloket/terms/>
PREFIX void:<http://rdfs.org/ns/void#>
PREFIX overheidbm:<http://standaarden.overheid.nl/bm/terms/>
PREFIX ow:<http://standaarden.overheid.nl/owms/terms>
PREFIX dcterms:<http://purl.org/dc/terms/>
PREFIX rdfs:<http://www.w3.org/2000/01/rdf-schema#>
PREFIX overheidvg:<http://standaarden.overheid.nl/vergunningen/terms/>
PREFIX overheidproduct:<http://standaarden.overheid.nl/product/terms/>
PREFIX overheidrg:<http://standaarden.overheid.nl/cvdr/terms/>
PREFIX xsd:<http://www.w3.org/2001/XMLSchema#>
PREFIX owl:<http://www.w3.org/2002/07/owl#>
PREFIX overheid:<http://standaarden.overheid.nl/owms/terms/>
PREFIX rdf:<http://www.w3.org/1999/02/22-rdf-syntax-ns#>
PREFIX scovo:<http://purl.org/NET/scovo#>
PREFIX overheidvac:<http://standaarden.overheid.nl/vac/terms/>
PREFIX skos:<http://www.w3.org/2004/02/skos/core#>
PREFIX overheidop:<http://standaarden.overheid.nl/op/terms/>
CONSTRUCT {
    <http://standaarden.overheid.nl/owms/terms/OWMSdataset> a <http://rdfs.org/ns/void#Dataset> .
    <http://standaarden.overheid.nl/owms/terms/OWMSdataset> dcterms:creator <http://standaarden.overheid.nl/owms/terms/ICTU> .
    <http://standaarden.overheid.nl/owms/terms/OWMSdataset> dcterms:description "De dataset van de Nederlandse overheid"@nl .
    <http://standaarden.overheid.nl/owms/terms/OWMSdataset> dcterms:issued ?date .
    <http://standaarden.overheid.nl/owms/terms/OWMSdataset> dcterms:license <http://www.gnu.org/copuleft/fdl.html> .
    <http://standaarden.overheid.nl/owms/terms/OWMSdataset> dcterms:title "OWMS dataset" .
    <http://standaarden.overheid.nl/owms/terms/OWMSdataset> <http://rdfs.org/ns/void#Feature> _:b0 .
    _:b0 dcterms:format "text/rdf+n3" .
    <http://standaarden.overheid.nl/owms/terms/OWMSdataset> <http://rdfs.org/ns/void#Feature> _:b1 .
    _:b1 dcterms:format "application/rdf+xml" .
    <http://standaarden.overheid.nl/owms/terms/OWMSdataset> <http://rdfs.org/ns/void#exampleResource> <http://standaarden.overheid.nl/owms/terms/Schiermonnikoog_(gemeente)> .
    <http://standaarden.overheid.nl/owms/terms/OWMSdataset> <http://rdfs.org/ns/void#exampleResource> <http://standaarden.overheid.nl/owms/terms/omgevingsvergunning> .
    <http://standaarden.overheid.nl/owms/terms/OWMSdataset> <http://rdfs.org/ns/void#exampleResource> <http://standaarden.overheid.nl/owms/terms/ROM_Rijnmond> .
    <http://standaarden.overheid.nl/owms/terms/OWMSdataset> <http://rdfs.org/ns/void#statItem> _:b2 .
    _:b2 rdf:value ?agents .
    _:b2 <http://purl.org/NET/scovo#dimension> <http://rdfs.org/ns/void#numberOfResources> .
    _:b2 <http://purl.org/NET/scovo#dimension> dcterms:Agent .
    <http://standaarden.overheid.nl/owms/terms/OWMSdataset> <http://rdfs.org/ns/void#statItem> _:b3 .
    _:b3 rdf:value ?concepts .
    _:b3 <http://purl.org/NET/scovo#dimension> <http://rdfs.org/ns/void#numberOfResources> .
    _:b3 <http://purl.org/NET/scovo#dimension> <http://www.w3.org/2004/02/skos/core#Concept> .
    <http://standaarden.overheid.nl/owms/terms/OWMSdataset> <http://rdfs.org/ns/void#statItem> _:b4 .
    _:b4 rdf:value ?geos .
    _:b4 <http://purl.org/NET/scovo#dimension> <http://rdfs.org/ns/void#numberOfResources> .
    _:b4 <http://purl.org/NET/scovo#dimension> dcterms:Location .
    <http://standaarden.overheid.nl/owms/terms/OWMSdataset> <http://rdfs.org/ns/void#statItem> _:b5 .
    _:b5 rdf:value ?triples .
    _:b5 <http://purl.org/NET/scovo#dimension> <http://rdfs.org/ns/void#numberOfTriples> .
    <http://standaarden.overheid.nl/owms/terms/OWMSdataset> <http://rdfs.org/ns/void#statItem> _:b6 .
    _:b6 rdf:value ?subjects .
    _:b6 <http://purl.org/NET/scovo#dimension> <http://rdfs.org/ns/void#numberOfDistinctSubjects> .
    <http://standaarden.overheid.nl/owms/terms/OWMSdataset> <http://rdfs.org/ns/void#statItem> _:b7 .
    _:b7 rdf:value ?objects .
    _:b7 <http://purl.org/NET/scovo#dimension> <http://rdfs.org/ns/void#numberOfDistinctObjects> .
    <http://standaarden.overheid.nl/owms/terms/OWMSdataset> <http://rdfs.org/ns/void#uriRegexPattern> "http://standaarden.overheid.nl/owms/terms/.+" .
    <http://standaarden.overheid.nl/owms/terms/OWMSdataset> <http://rdfs.org/ns/void#vocabulary> <http://www.w3.org/2008/05/skos-xl#> .
    <http://standaarden.overheid.nl/owms/terms/OWMSdataset> <http://rdfs.org/ns/void#vocabulary> <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
    <http://standaarden.overheid.nl/owms/terms/OWMSdataset> <http://rdfs.org/ns/void#vocabulary> <http://www.w3.org/2000/01/rdf-schema#> .
    <http://standaarden.overheid.nl/owms/terms/OWMSdataset> <http://rdfs.org/ns/void#vocabulary> <http://xmlns.com/foaf/0.1/> .
    <http://standaarden.overheid.nl/owms/terms/OWMSdataset> <http://rdfs.org/ns/void#vocabulary> <http://www.w3.org/2004/02/skos/core#> .
    <http://standaarden.overheid.nl/owms/terms/OWMSdataset> <http://rdfs.org/ns/void#vocabulary> <http://purl.org/dc/terms/> .
    <http://standaarden.overheid.nl/owms/terms/OWMSdataset> <http://rdfs.org/ns/void#vocabulary> xsd: .
    <http://standaarden.overheid.nl/owms/terms/OWMSdataset> <http://rdfs.org/ns/void#vocabulary> <http://standaarden.overheid.nl/owms/terms/> .
    <http://standaarden.overheid.nl/owms/terms/OWMSdataset> <http://rdfs.org/ns/void#vocabulary> <http://www.w3.org/2002/07/owl#> .
}

WHERE {
{SELECT ( count(?a) as ?geos )
WHERE {
?a a ?type.
?type rdfs:subClassOf+ dcterms:Location.}}
{SELECT ( count(?b) as ?agents )
WHERE {
?b a ?type.
?type rdfs:subClassOf+ dcterms:Agent.}}
{SELECT ( count(?c) as ?concepts )
WHERE {
?c a ?type.
?type rdfs:subClassOf+ skos:Concept.}}
{SELECT (count (*) as ?triples) (count(distinct ?a) as ?subjects) (count (distinct ?c) as ?objects)
WHERE
{
?a ?b ?c.
}}
}