Can't insert multiple values using drupal_execute() (drupal_prepare_form() problem?)

I have 2 fields (field_a nodereference and field_b text) in a multigroup (group_test). My multigroup can accept an unlimited # of values.

When I run the following test code only the first fieldgroup entry is saved (field_a = nid 391, field_b = test 1), the second entry is completely ignored:

require("modules/node/node.pages.inc");
$node = node_load(392);
$form_state['values'] = (array)$node;
$form_state['values']['field_a'][0]['nid'] = '391';
$form_state['values']['field_b'][0]['value'] = 'test 1';
$form_state['values']['field_a'][1]['nid'] = '391';
$form_state['values']['field_b'][1]['value'] = 'test 2';
$form_state['values']['op'] = 'Save';
drupal_execute('mynodetype_node_form',$form_state, $node);

A little investigation seems to indicate that the problem occurs somehow when drupal_prepare_form() runs (inside of drupal_execute()). The form prepared by drupal_prepare_form generates a group_test array with only as many delta entries as the node had before being edited.

You may wonder why use drupal_execute? The above code is just a simplified version of what the Services module uses to save nodes so I really need it to work. For reference, below is an exact copy of the relevant code from the Services module :

function node_service_save($edit) {
  // Load the required includes for drupal_execute
  module_load_include('inc', 'node', 'node.pages');
  $nid = NULL;
  $edit = (array) $edit;
  if ($edit['nid']) {
    $node = node_load($edit['nid']);
    if ($node->nid) {
      // Setup form_state.
      $form_state = array();
      $form_state['values'] = (array) $edit;
      $form_state['values']['op'] = t('Save');

      // Later on in the chain node_form seems to expect a copy of
      // the old node object.
      $form_state['node'] = (array) $node;

      $ret = drupal_execute($node->type .'_node_form', $form_state, (object)$node);

      // If the node is immediately reloaded after update, it will
      // load the OLD cached version.
      node_load(0, NULL, TRUE);

      // Set $nid, so it can be returned
      $nid = $node->nid;
    }
    else {
      return services_error(t('Node not found'), 401);
    }
  }
  else {
    // If the submitted node has no author (for instance if it was
    // created programmatically by hand) then set the currently logged 
    // in user as the author. 
    if (!$edit->uid) {
      global $user;
      $edit->uid = $user->uid;
    }

    // Setup form_state
    $form_state = array();
    $form_state['values'] = (array) $edit;
    $form_state['values']['op'] = t('Save');

    $ret = drupal_execute($edit['type'] .'_node_form', $form_state, (object)$edit);

    // Fetch $nid out of $form_state
    $nid = $form_state['nid'];
  }
  if ($errors = form_get_errors()) {
    return services_error(implode("\n", $errors), 401);
  }
  return $nid;
}