filter query variables across two named graphs in the same dataset

[Leo Papa]

Hello fellas,

I am trying to make my mind around this issue and I still cannot figure it out.

I have Graph g1 (g1.n3 attached) and Graph g2 (g2.n3 attached), where g2 has 
the same structure of g1 
and share a subset of entities (CODE) 

- (code1, code2) in g1 

- 
(code1, code2, code3, code4) in g2

I create Jena DatasetGraph with g1 and g2 and a query (query.txt attached)

DatasetGraph dsg = DatasetGraphFactory.createMem(); 

dsg.addGraph("http://example.org/g1";, g1); 

dsg.addGraph("http://example.org/g2"; g2); 

QueryExecutionFactory.create(query,DatasetFactory.create(dsg)).execConstruct().getGraph();
I want to achieve a query result that CONSTRUCT the graph/model constraining on 
the CODEs in g2 that are not in g1, 
hence:
CONSTRUCT { ?codeG2 ?p ?x . ?ce2 ?r ?y . } 

WHERE 

{ GRAPH <http://example.org/g1> 

    { ?codeG1 <http://example.org/code/classifies> ?ce1} 

 GRAPH <http://example.org/g2> 

    { ?codeG2 <http://example.org/code/classifies> ?ce2 . 

      ?ce2 ?r ?y. ?codeG2 ?p ?x } 

      FILTER(?codeG2 != ?codeG1) 
} 


The result I expect should be (g_result.n3 attached):
Graph: http://example.org/result 

@prefix code: <http://example.org/code/> . 

@prefix ce:   <http://example.org/codedentity/> . 

@prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> .code:nff code:classifies 
ce:1221-32. 

ce:nff rdfs:label "nff" . 

ce:1221-32 rdfs:label "1221-32" . 

code:nff a code:Code.
code:cca code:classifies ce:5541-32. 

ce:cca rdfs:label "cca" . 

ce:5541-32 rdfs:label "5541-32" .code:cca a code:Code. 

Instead of the expected/wanted result I have  a graph that contains the correct 
CONSTRUCT pattern, but does not FILTER according the condition. What's more 
weird is that the filter 
will work for (?codeG2 = ?codeG1) but not the !=.
Where am I doing it wrong? Someone can try to test this?

Thanks a lot
CONSTRUCT { ?codeG2 ?p ?x . ?ce2 ?r ?y . } 
WHERE { 
GRAPH <http://example.org/g1> 
        { ?codeG1 <http://example.org/code/classifies> ?ce1} 
GRAPH <http://example.org/g2> 
        { ?codeG2 <http://example.org/code/classifies> ?ce2 . 
          ?ce2 ?r ?y. ?codeG2 ?p ?x 
        } 
        FILTER(?codeG2 != ?codeG1)
} 
@prefix code: <http://example.org/code/> . 
@prefix ce: <http://example.org/codedentity/> .
@prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> .

code:afr code:classifies ce:3455-32. 
ce:afr rdfs:label "afr" . 
ce:3455-32 rdfs:label "3455-32" . 
code:afr a code:Code.

code:tfs code:classifies ce:9983-32. 
ce:tfs rdfs:label "tfs" . 
ce:9983-32 rdfs:label "9983-32" . 
code:tfs a code:Code. 
@prefix code: <http://example.org/code/> . 
@prefix ce: <http://example.org/codedentity/> . 
@prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> .

code:afr code:classifies ce:4411-32. 
#(notice same CODE BUT DIFFERENT CE from g1) 
ce:afr rdfs:label "afr" . 
ce:4411-32 rdfs:label "4411-32" . 
ce:afr a ce:Code.

code:tfs code:classifies ce:7601-32. 
#(notice same CODE BUT DIFFERENT CE from g1) 
ce:tfs rdfs:label "tfs" . 
ce:7601-32 rdfs:label "7601-32" . 
ce:tfs a ce:Code.

code:nff code:classifies ce:1221-32. 
ce:nff rdfs:label "nff" . 
ce:1221-32 rdfs:label "1221-32" . 
ce:nff a ce:Code.

code:cca code:classifies ce:5541-32. 
ce:cca rdfs:label "cca" . 
ce:5541-32 rdfs:label "5541-32" . 
ce:cca a ce:Code
@prefix code: <http://example.org/code/> .
@prefix ce: <http://example.org/codedentity/> .
@prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> .

code:nff code:classifies ce:1221-32.
ce:nff rdfs:label "nff" .
ce:1221-32 rdfs:label "1221-32" .
code:nff a code:Code.

code:cca code:classifies ce:5541-32.
ce:cca rdfs:label "cca" .
ce:5541-32 rdfs:label "5541-32" .
code:cca a code:Code.