Plesk 11.x / Windows :: Additional Directives - PHP Extension

I'm running PP 11.0.9 on Windows 2008 R2 and I've tried to add a PHP extension to the additional directives box of the PHP settings page (domain), however this has no effect on PHP. If I add the extension manually to PHP.ini then it works fine, but adding it to the "additional directives" box does nothing - I've confirmed this on 3 different servers, it does the same on all of them.

I did some searching and it looks like others have this problem too: [URL] ....