Frank Goossens on "[Plugin: Autoptimize] CSS and JS files are ignored during minification"

That's weird; AO depends on content_url() to determine the URL used for the absolute URL's (which are required, as the font URL is often relative to the URL of the original stylesheet which get aggregated to an autoptimize-URL), cfr. these lines of code from autoptimize.php and from the fixurl-function in autoptimizeStyles.php;

define('AUTOPTIMIZE_WP_CONTENT_URL',content_url());
define('AUTOPTIMIZE_WP_ROOT_URL',str_replace('/wp-content','',AUTOPTIMIZE_WP_CONTENT_URL));
$newurl = AUTOPTIMIZE_WP_ROOT_URL.str_replace('//','/',$dir.'/'.$url);

The result of content_url() should indeed start with https:// if is_ssl() (yet another WP core function) is true.

So the question is; why does your site (and the HTTPS plugin) not return an HTTPS-url when content_url() is called?