javascript - poltergeist: jquery ajax does not accept one of two equals responses

working with symfony 2.8 I'm trying to make an AJAX call to a controller that returns a JSON that contains one array.

CONTROLER:

public function seguimientoAction(Request $request){
    $idUnico = $request->query->get('plan-id'); 
    $em = $this->getDoctrine()->getManager();
    $idPlan =  $em->getRepository('UsuariosBundle:Plan')->findOneByIdUnico($idUnico);
    $seguimientos = $em->getRepository('UsuariosBundle:Seguimiento')->findByPlan($idPlan);


    $contenedor = array();
    foreach ($seguimientos as $seguimiento){
        $fecha = $seguimiento->getFecha();
        $fecha = $fecha->format('d-m-y');

        $peso = $seguimiento->getPeso();

        $contenedor[] = ['fecha' => $fecha, 'peso' => $peso];
    }

    return new JsonResponse($contenedor);
 }
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JAVASCRIPT:

var selectorSeguimiento = $(".plan-seguimiento"); 

    selectorSeguimiento.click(function(event){
        event.preventDefault();

        var planid = $(this).attr("data-id");

            $.ajax({
                type: "POST",
                dataType: 'json',
                ContentType: 'application/json',
                url: Routing.generate('usuarios_dashboard_planes_seguimiento', {planid}),
                success: function(data){
                    console.log(data);
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If I write the url that call the controller in the browser I can see this:

[{"fecha":"26-02-18","peso":"67.0"},{"fecha":"28-02-18","peso":"66.0"},{"fecha":"03-03-18","peso":"64.0"}]

So, I understand that the problem is that javascript doesn't take the data..., but... ...If I change the array in controller and I make an identical array "artesanal way"...

CONTROLER(test):

{-code-3}   
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...I receive this:

Array(3) [ {…}, {…}, {…} ] localhost:90:25

Array(3) [ "67.0", "66.0", "64.0" ] localhost:99:25

Array(3) [ "26-02-18", "28-02-18", "03-03-18" ] localhost:100:25

And for additional check, I turn back to call with the browser and I obtain this:

[{"fecha":"26-02-18","peso":"67.0"},{"fecha":"28-02-18","peso":"66.0"},{"fecha":"03-03-18","peso":"64.0"}]

Exactly the same that the other case

So, the question is, Why does jquery ajax method "likes" the response with the second array and it "dislikes" the first one (being the same array)?

EDIT: Here you can see a var_dump() of the arrays before the json_encode():

Array $contenedor

{-code-4}
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Array $miarray

{-code-4}
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