do_shortcode returns nothing in AJAX call

I can't figure this one out. The same shortcode renders in the template file (non AJAX).

But via AJAX (code below) returns the err message.

$return = array();
if ( $formcode = do_shortcode("[mb_frontend_form id='listing-actions' post_fields='title,content']") ) {
	$return['html'] = $formcode;
} else {
	$return['err'] = 'No form returned.';
}
echo json_encode($return);
die;

Is this a known limitation? Any tips appreciated.