Symfony post submit event doesnt modify dynamic field - symfony

Question

Here is my case:
I have two fields in a form, the second field values depends on the value selected on the first field.
I'm doing this to modify the second field:
$builder->add('firstField',EntityType::class, array(class => First::class));
$formModifier = function(FormInterface $formBuilder, $data = null){
if($data != null){
$formBuilder->add('secondField', EntityType::class, array(
class => Second::class,
'query_builder' => function (EntityRepository $er) use ($data) {
return $er->createQueryBuilder('s')
->where('s.field = :data')
->setParameter('data', $data)
;
});
} else {
$formBUilder->add('secondField', HiddenType::class, array('required' => false);
}
}
$builder->addEventListener(
FormEvents::PRE_SET_DATA,
function (FormEvent $event) use ($formModifier) {
$data = $event->getData();
$formModifier($event->getForm(), $data->getData());
}
);
$builder->get('firstField')->addEventListener(
FormEvents::POST_SUBMIT,
function (FormEvent $event) use ($formModifier){
$data = $event->getForm()->getData();
$formModifier($event->getForm()->getParent(), $data);
}
);
Ajax:
var $firstField = $('#firstField');
$(document).on('change', '#firstField', function(){
var $form = $(this).closest('form');
var data = {};
data['id'] = $(this).val();
$.ajax({
url: $form.attr('action'),
type: $form.attr('method'),
data: data,
success: function (html) {
$('#secondField').html(
$(html).find('#secondField').html()
);
}
})
});
The data is sent correctly via ajax, but i never get the correct response because it doesn't ever enter in the POST_SUBMIT event listener as i tried to debug and use some var_dump to check it. Any help would be appreciated. Thanks.