This is not related to the problem here: https://piklist.com/support/topic/hidden-field-bugs/

The group add_more does not display correctly when a hidden field is included in a group.

Here is my erroneous code –

Title: Recipe Options
Post Type: recipe
Order: 1
Collapse: false
*/

piklist('field', array(	
  'type' => 'group'
  ,'add_more' => true
  ,'scope' => 'post_meta'
  ,'columns' => 12
  ,'fields' => array(
    array(
      'field' => 'recipe_id'
      ,'type' => 'hidden'
      ,'value' => ''
    )
    ,array(
      'field' => 'recipe_title'
      ,'columns' => 12
      ,'label' => __('Recipe Title','ideas_solutions')
      ,'type' => 'text'
      ,'required' => true
      ,'columns' => 12
    )	
  )
));

This is working code –

Title: Recipe Options
Post Type: recipe
Order: 1
Collapse: false
*/

piklist('field', array(	
  'type' => 'group'
  ,'add_more' => true
  ,'scope' => 'post_meta'
  ,'columns' => 12
  ,'fields' => array(array(
      'field' => 'recipe_title'
      ,'columns' => 12
      ,'label' => __('Recipe Title','ideas_solutions')
      ,'type' => 'text'
      ,'required' => true
      ,'columns' => 12
    )	
  )
));

It would be nice to have hidden fields in an add_more so I can filter the meta data on insert/update