PrimitiveTypeConstraintValidator error when using #element_validate on a Date Range field

When trying to implements an #element_validate callback on the start date element of a Date Range field :

$form['field_candidates_unavailability']['widget'][0]['value']['#element_validate'][] = [$this, 'validateUnavailabiltyDates'];

I get a "This value should be of the correct primitive type." error on validation. The value has not been modified, just a simple diff test with the today date :

$start_value = $form_state->getValue('field_candidates_unavailability');
/** @var \Drupal\Core\Datetime\DrupalDateTime $start_date */
$start_date = $start_value[0]['value']['object'];
$today = new DrupalDateTime('now', drupal_get_user_timezone());
datetime_date_default_time($today);
$diff = $today->diff($start_date);
if ($diff->days == 1 && $diff->invert == 1) {
  // Start date is anterior to today, throw an error.
  $form_state->setErrorByName(implode('][', $element['#parents']), 'The start date must be posterior to today.');
}

I can see that PrimitiveTypeConstraintValidator::validate method get an array for the field $value, instead of a date in string format.

I've to do this weird trick in the #element_validate callback to make this PrimitiveTypeConstraintValidator works :

$form_state->setValue($element['#parents'], $start_value[0]['value']['date']);