Flattening problem with set variables

[jnmonette]

Solving the following model produces two solutions: one with "obj = true" and one with "obj= false", while there should really only be the one with "obj = true" as which is constrained to be non-empty.

var set of 1..1: which;
constraint 1 in which;

var bool: obj;
constraint obj = exists([true | s in which]);

solve satisfy;

I could observe the behaviour with 2.0.8 and 2.0.10. Surprisingly enough, the "exists" construct translates into an "array_bool_and", would it be the problem?