Steps to reproduce the issue

In admin side code:

use \Joomla\CMS\Router\Router;
$router = Router::getInstance('site');
$routedUrl = $router->build('index.php?option=com_content&view=article&id=1&Itemid=1');
$routedUrlAsString = $routedUrl->toString();

Expected result

For menu item with alias 'foo' pointing at article 'bar':

https://my.site.tld/foo/bar

Actual result

https://my.site.tld/administrator/foo/bar

System information (as much as possible)

Additional comments

Method SiteRouter::build($url) prefixes the constructed SEF url with \JUri::base(true). When invoked from within admin code, JUri::base() returns an uri containing '/administrator', which is not the intention for a site SEF url.