Currently if a column is not visible, it's not sortable. See notes at build_order_by().

While in View Issues it's not a problem because it makes no sense to sort by an invisible column, in RSS the default column to sort by is date_submitted which is not visible by default. The RSS feed is always sorted in the default order (i.e. last updated).

/* issues_rss.php */
if( $f_sort === 'update' ) {
    $c_sort_field = 'last_updated';
} else {
    $c_sort_field = 'date_submitted';  // no effect due to invisibility
}

A temporary fix is to use id instead of date_submitted. But the real question is should it be allowed to sort by an invisible column?