I'm trying for hours now to find the error moving back and forth. I have a custom post type. Everything works fine until I add my custom meta box. Now suddenly the slug gets the title of one of `$post->ID == '21'` child pages instead its own title when creating a new custom post. But when I remove my custom meta box again. Everything works fine. I may have tracked it down to the `foreach` but I can't see what's wrong there.

Besides this `%postname%` error everything's fine. The custom field gets saved correctly. I even can edit the slug manually. But that's not how it is supposed to be. Please help!

    add_action( 'init', 'cpt_init_cases' );
    function cpt_init_cases() {
      register_post_type( 'case', array(
        'labels' => array(
          'name' => __( 'Cases' ),
          'singular_name' => __( 'Case' )),
        'description' => 'Create cases to be displayed on single customer child pages',
        'public' => true,
        'menu_position' => 6,
        'rewrite' => array('slug' => 'case'),
        'supports' => array('title', 'editor', 'thumbnail', 'custom-fields')
      ));
    }
    
    /*** when I delete everything below this line slugging works as expected ***/
    add_action('admin_init', 'admin_init');
    function admin_init(){
        add_meta_box('refInfo-meta', 'Reference', 'reference_meta_callback', 'case', 'side', 'high');
    }  
    
    function reference_meta_callback( $post ){
    
      global $post;
    
      $custom = get_post_custom($post->ID);
      if( isset($custom['reference_id'][0]) ) {
        $reference_id = $custom['reference_id'][0];
      } else {
        $reference_id = '0';
      }
    
      wp_nonce_field( 'my_meta_box_nonce', 'meta_box_nonce' );
    
      ?>
        <form action="<?php bloginfo('url'); ?>" method="get">
          <select name="ref_id" id="ref_id">
            <option value="0" <?php selected($reference_id, '0'); ?>>- choose client -</option>
    
          <?php
          $args = array( 'numberposts' => -1, 'post_type' => 'page', 'post_parent' => 21);
          $posts = get_posts( $args );
          foreach( $posts as $post ) : setup_postdata( $post ); ?>
            <option value="<?php echo $post->ID; ?>" <?php selected($reference_id, $post->ID); ?>><?php the_title(); ?></option>
          <?php endforeach; ?>
    
          </select>
        </form>
      <?php
    }  
    
    add_action( 'save_post', 'save_reference_id' );  
    function save_reference_id( $post_id ) {
    
      global $post;
    
      if( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE ) return;
    
      if( !isset( $_POST['meta_box_nonce'] ) || !wp_verify_nonce( $_POST['meta_box_nonce'], 'my_meta_box_nonce' ) ) return;
    
      if( isset($_POST['ref_id']) ) {
        update_post_meta($post->ID, 'reference_id', $_POST['ref_id']);
      }
    }