The problem here is that actually the addHeader() function doesn't have a 4th parameter to pass the ID. The ID is hardcoded to use the form id together with the header string and the name you provide as the 1st parameter instead:
public function addHeader( $lang, $after=NULL, $tab=NULL )
{
/* Place the input into the correct position */
$this->_insert( \IPS\Theme::i()->getTemplate( 'forms', 'core' )->header( $lang, "{$this->id}_header_{$lang}" ), NULL, $tab, $after );
}
So yeah, not exactly a bug. It would be nice it IPS allows to pass a custom ID as 4th parameter just like the other form helpers though.