sdbquery: FILTER within FILTER NOT EXIST has no effect

@prefix skos: <http://www.w3.org/2004/02/skos/core#>.
@prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>.
@prefix ex: <http://example.org/>.

# source graph
ex:src {
        ex:src1 rdfs:label "label1" .
        ex:src2 rdfs:label "label2" .
}

# destination graph
ex:dst {
        ex:dst1 skos:prefLabel "label1" .
        ex:dst2 skos:altLabel "label2" .
}

PREFIX rdfs: <http://www.w3.org/2000/01/rdf-schema#>
PREFIX skos: <http://www.w3.org/2004/02/skos/core#>
PREFIX ex: <http://example.org/>

SELECT * {
        GRAPH ex:src {
                ?c1 rdfs:label ?label .
        }

        FILTER NOT EXISTS {
                GRAPH ex:dst {
                        ?c2 ?anyprop ?label .
                        FILTER (?anyprop != skos:prefLabel)
                }
        }
}

----------------------
| c1      | label    |
======================
| ex:src1 | "label1" |
----------------------

PREFIX  skos: <http://www.w3.org/2004/02/skos/core#>
PREFIX  rdfs: <http://www.w3.org/2000/01/rdf-schema#>
PREFIX  ex:   <http://example.org/>

SELECT  *
WHERE
  { GRAPH ex:src
      { ?c1  rdfs:label  ?label }
    FILTER NOT EXISTS { GRAPH ex:dst
                          { ?c2  ?anyprop  ?label
                            FILTER ( ?anyprop != skos:prefLabel )
                          }
                      }
  }
- - - - - - - - - - - - - -
(prefix ((rdfs: <http://www.w3.org/2000/01/rdf-schema#>))
  (filter (notexists
             (filter (!= ?anyprop <http://www.w3.org/2004/02/skos/core#prefLabel>)
               (SQL '''SqlSelectBlock/Q_1
                     Q_1.g = -273327510643043486
                   Table Q_1             -- ex:dst ?c2 ?anyprop ?label''')
             ))
    (SQL '''SqlSelectBlock/S_1           -- V_1=?c1 V_2=?label
          R_1.lex/V_1_lex R_1.datatype/V_1_datatype R_1.lang/V_1_lang R_1.type/V_1_type 
          R_2.lex/V_2_lex R_2.datatype/V_2_datatype R_2.lang/V_2_lang R_2.type/V_2_type
        Join/left outer
          Join/left outer
            SqlSelectBlock/Q_2           -- ?c1:(Q_2.s=>Q_2.X_1) ?label:(Q_2.o=>Q_2.X_2)
                Q_2.s/X_1 Q_2.o/X_2
                Q_2.g = -317857793313676831
                Q_2.p = 6454844767405606854
              Table Q_2                  -- ex:src ?c1 rdfs:label ?label
            Table R_1                    -- Var: ?c1
            Condition Q_2.X_1 = R_1.hash
          Table R_2                      -- Var: ?label
          Condition Q_2.X_2 = R_2.hash''')
  ))
SELECT                                   -- V_1=?c1 V_2=?label
  R_1.lex AS V_1_lex, R_1.datatype AS V_1_datatype, R_1.lang AS V_1_lang, R_1.type AS V_1_type, 
  R_2.lex AS V_2_lex, R_2.datatype AS V_2_datatype, R_2.lang AS V_2_lang, R_2.type AS V_2_type
FROM
    ( SELECT                             -- ?c1:(Q_2.s=>Q_2.X_1) ?label:(Q_2.o=>Q_2.X_2)
        Q_2.s AS X_1, Q_2.o AS X_2
      FROM Quads AS Q_2                  -- ex:src ?c1 rdfs:label ?label
      WHERE ( Q_2.g = -317857793313676831 -- Const: ex:src
         AND Q_2.p = 6454844767405606854 -- Const: rdfs:label
         )
    ) AS Q_2                             -- ?c1:(Q_2.s=>Q_2.X_1) ?label:(Q_2.o=>Q_2.X_2)
  LEFT OUTER JOIN
    Nodes AS R_1                         -- Var: ?c1
  ON ( Q_2.X_1 = R_1.hash )
  LEFT OUTER JOIN
    Nodes AS R_2                         -- Var: ?label
  ON ( Q_2.X_2 = R_2.hash )

SELECT                                   -- ?c2:(Q_3.s=>Q_3.X_1) ?anyprop:(Q_3.p=>Q_3.X_2)
  Q_3.s AS X_1, Q_3.p AS X_2
FROM Quads AS Q_3                        -- ex:dst ?c2 ?anyprop "label1"
WHERE ( Q_3.g = -273327510643043486      -- Const: ex:dst
   AND Q_3.o = -9201284561348719236      -- Const: "label1"
   )

SELECT                                   -- ?c2:(Q_4.s=>Q_4.X_1) ?anyprop:(Q_4.p=>Q_4.X_2)
  Q_4.s AS X_1, Q_4.p AS X_2
FROM Quads AS Q_4                        -- ex:dst ?c2 ?anyprop "label2"
WHERE ( Q_4.g = -273327510643043486      -- Const: ex:dst
   AND Q_4.o = 2153590109465208943       -- Const: "label2"
   )

----------------------
| c1      | label    |
======================
| ex:src1 | "label1" |
| ex:src2 | "label2" |
----------------------