I am not sure that your parser is catching the right type of error with this audit?

In my case, I have an adaptive image plugin called ShortPixel AI (https://wordpress.org/plugins/shortpixel-adaptive-images/) that proxies images, CSS, JS through their CDN.

The result looks like this: https://images.DOMAIN.com/spai/q_lossless+ret_img+v_5/https://www.DOMAIN.com/wp-content/cache/autoptimize/css/autoptimize_d4e82ce88fd9b17f32a12ca585359015.css