behavior of disabling constraint_mode from subclass

Hello experts,
we found some inconsistent behavior between qsim and ncsim when trying to enable/disable constraint_mode from subclass. Here is the code sample:


class cls;
    rand int a;
    rand int b;
    rand int c;

    constraint c_a {a == -1;}
    constraint c_b {b == -1;}
    constraint c_c {c == 1;}

    function new();
    endfunction

    virtual function void do_rand();
      $display("Before randomize ...");
      $display("  a = %0d", a);
      $display("  b = %0d", b);
      $display("  c = %0d", c);


      this.constraint_mode(0);   //how many constraint blocks are disable? cls::c_a, cls::c_b, cls::c_c,  cls_e::c_a, cls_e::c_b
      this.c_a.constraint_mode(1); // which is enabled?
      this.c_b.constraint_mode(1); // which is enabled?

      assert(this.randomize(a,b) with {1;});

      this.constraint_mode(1);
      $display("After randomize ...");
      $display("  a = %0d", a);
      $display("  b = %0d", b);
      $display("  c = %0d", c);
    endfunction
endclass

class cls_e extends cls;
    constraint c_a { a==8000; }
    constraint c_b { b==40; }

endclass

module test();
    cls_e cls_inst;
    initial begin
      cls_inst = new;
      cls_inst.do_rand();
    end
endmodule


In qsim, cls_e::c_a and cls_e::c_b are working; but not in ncsim...
Is the behavior of the above three "constraint_mode()" statements clearly defined in spec?

-Xiaofei