Well, I’ve found a way to do it.
Here it is:
(all the code is in the form’s file)

  $form_id = $_POST['_']['form_id']; 
  // if we don't have a form posted OR if the form posted contains errors: 
  if( !$form_id || !empty( Piklist_Validate::notices( $form_id, true) )  ) { 
    //display form
    piklist('field', array(
      'type' => 'text
      ,'scope' => 'contact'
      ...
    ));
    ....
  } else {
  ?>
  <h3>Your request:</h3>
  <?php 
	foreach( $_POST['_contact'] as $label => $value ) {
		echo '<p><strong>' . $label . ':</strong> ' . $value . '</p>';
	}
	?>
	<a href="<?= the_permalink(); ?>" class="btn btn-default">Go back to Contact form</a>
	<?php
  }

Hi there,

Just encountered the same problem.
I looked around, and if in class-piklist-form.php line 3775 -> public static function notices
you add && empty( Piklist_Validate::notices( self::$form_id, true) ) in the if statement it sorts the problem.

Full code:

  public static function notices($form_id = null)
  {
    if (!empty(self::$form_submission)
        && self::$form_id
        && self::$form_id == $form_id
        && isset(self::$forms[self::$form_id])
        && !empty(self::$forms[self::$form_id]['data']['message'])
        && empty( Piklist_Validate::notices( self::$form_id, true) ) // Checks if there are any errors
      )
    {
      piklist::render('shared/notice', array(
        'id' => 'piklist_form_notice'
        ,'notice_type' => 'update'
        ,'dismiss' => is_admin()
        ,'content' => self::$forms[self::$form_id]['data']['message'] 
      ));
    }
  }

At least it works for the frontend forms.
Not sure if it would have any effect somewhere else!