drupal_set_message() content does not display, even with nocache=1 in the URL

I'm seeing this both in the sidebar block provided by the constant contact module and on a custom form I created. My custom form isn't doing anything specific with redirect. The user just stays on the form page with the submission results being displayed via drupal_set_message().

The form validation messages do work correctly with boost enabled. It's just the submission result message that does not display. I have confirmed that the form submission does add the user to the mailing list even though it doesn't display the result message.

The result message does display on the form if we disable boost or if the user is logged into the site.

The actual form is at http://www.preparemaine.org/get-updates if seeing it helps.

Some additional information on this. Just disabling the boost caching on the performance page is not enough to get the drupal_set_message() calls to work. I had to completely uninstall the boost module from the site in order for these messages to display on our site.

This is also impacting the form_set_error() call too.

Below is the code from our form submit handler. None of the calls to drupal_set_message() or form_set_error() display an error message when the boost module is installed on our site.

function mcee_constant_contact_form_submit($form, &$form_state) {
  $user_email = $form_state['values']['email'];
  $fields['FirstName'] = $form_state['values']['first_name'];
  $fields['LastName'] = $form_state['values']['last_name'];
  $fields['PostalCode'] = $form_state['values']['zipcode'];

  $cc = constant_contact_create_object();
  $cc->set_action_type('contact'); /* important, tell CC that the contact made this action */

  $contact = $cc->query_contacts($user_email);
  if ($contact) {
    form_set_error('email', t('Your email address is already subscribed'));
    return;
  }

  $i_am = '';
  foreach ($form_state['values']['i_am'] as $key => $value) {
    if ($value) {
      if (strlen($i_am) > 0) $i_am .= ", ";
      $i_am .= $value;
    }
  }

  $fields['CustomField1'] = $i_am;

  $lists = $form_state['values']['lists'];
  $newlists = array();
  foreach($lists as $list_id => $enabled):
    if (intval($enabled) !== 0):
      $list = $cc->get_list($list_id);
      $newlists[$list_id] = $list['Name'];
    endif;
  endforeach;
  $lists = $newlists;


  $lists = array_keys($lists);
  $status = $cc->create_contact($user_email, $lists, $fields);
  if ($status) {
    drupal_set_message(t('Thank you for joining our list'));
  }
  else {
    form_set_error('email', t('An error occurred while trying to add you to the selected lists.'));
  }
}