When a user with the f_delete permission attempts to permanently delete a post, the option is ignored and the post is soft-deleted instead. The culprit seems to be the following in posting.php which only checks for m_delete before switching to soft-delete mode.

// If the user is not allowed to delete the post, we try to soft delete it, so we overwrite the mode here.

if ($mode == 'delete' && (($confirm && !$request->is_set_post('delete_permanent')) || !$auth->acl_get('m_delete', $forum_id)))

{

	$mode = 'soft_delete';

}