Consider using `<link rel=preload>` to prioritize fetching resources that are currently requested later in page load.

I have the same problem, this is happening because the regex in the DeferCss class does not match the urls of sources.googleapis.com.
I gave you a custom solution

In my theme.libraries.ym i add:
//fonts.googleapis.com/css?family=Montserrat:400,500,600,700|Work+Sans:400,500,600,700: { type: external, attributes: { as: style, onload: "this.rel='stylesheet'" } }

And in mi theme.module i add:

/**
 * Implements hook_page_attachments_alter().
 *
 */
function theme_page_attachments_alter(array &$attachments) {
  $html = '<noscript><link rel="stylesheet" media="all" href="//fonts.googleapis.com/css?family=Montserrat:400,500,600,700|Work+Sans:400,500,600,700" /></noscript>';
  $attachments['#attached']["html_head"][] = [
    [
      '#type' => 'inline_template',
      '#template' => $html,
    ]
  ];
}

I base myself on the operation of the module html_head → .