Hidden steps values are not removed

For now my workaround was:

/**
 * Implements hook_webform_submission_form_alter().
 */
function mymodule_webform_submission_form_alter(array &$form, FormStateInterface $form_state, $form_id) {
  if (\Drupal::service('router.admin_context')->isAdminRoute()) {
    return FALSE;
  }

  if (!empty($form['actions']['wizard_prev']['#submit'])) {
    array_unshift(
      $form['actions']['wizard_prev']['#submit'],
      'mymodule_webform_submission_form_alter_wizard_prev_submit'
    );
  }
}

/**
 * Submit callback for the previous page submit.
 */
function mymodule_webform_submission_form_alter_wizard_prev_submit($form, FormStateInterface $form_state) {
  $current_page = $form_state->get('current_page');

  /** @var \Drupal\webform\WebformSubmissionForm $form_object */
  $form_object = $form_state->getFormObject();
  /** @var \Drupal\webform\WebformSubmissionInterface $webform_submission */
  $webform_submission = $form_object->getEntity();
  $webform = $webform_submission->getWebform();
  foreach ($webform->getElementsInitializedAndFlattened() as $key => $element) {
    if (!empty($element['#webform_parents']) && in_array($current_page, $element['#webform_parents'])) {
      // Clear value for elements of the current page.
      $form_state->setValue($key, '');
    }
  }
}