CssOptimizer silently drop the attached library when a CSS file @include an external CSS.

The logic here was originally added in:

commit 5d8cf1b865b1a926957bbc8802618dad08506fe0
Author: Gábor Hojtsy <gabor@hojtsy.hu>
Date:   Fri Dec 7 11:14:05 2007 +0000

    #113607 by Steven, chx, hass, catch and dvessel: proper inclusion of style sheets when/where @import is used

Looks like it was last fixed in #2936067: CSS aggregation fails on many variations of @import → which seems similar to your issue and added test coverage.

I've added your example google fonts URL to the existing unit test in case there was something in there that was confusing the regexp, and that test still passes.

I also did some manual testing with Olivero.

Added this diff:

diff --git a/core/themes/olivero/css/base/base.css b/core/themes/olivero/css/base/base.css
index 70471f960e..6fab540f31 100644
--- a/core/themes/olivero/css/base/base.css
+++ b/core/themes/olivero/css/base/base.css
@@ -1,3 +1,5 @@
+@charset "UTF-8";
+@import url("https://fonts.googleapis.com/css2?family=Fraunces:opsz,wght@9..144,400;9..144,600&display=swap");
 /*
  * DO NOT EDIT THIS FILE.
  * See the following change record for more information,

Then I checked the CSS aggregates (it was the second of two on the front page), and the first bit of the aggregate looks like this:

@import url("https://fonts.googleapis.com/css2?family=Fraunces:opsz,wght@9..144,400;9..144,600&display=swap");
/* @license GNU-GPL-2.0-or-later https://www.drupal.org/licensing/faq */
@font-face{font-family:metropolis;src:url(/core/themes/olivero/fonts/metropolis/Metropolis-Regular.woff2) format("woff2");font-weight:normal;font-style:normal;font-display:swap;}@font-face{font-family:metropolis;src:url(/core/themes/olivero/fonts/metropolis/Metropolis-Bold.woff2) format("woff2");font-weight:700;font-style:normal;font-display:swap;}@font-face{font-family:metropolis;src:url(/core/themes/olivero/fonts/metropolis/Metropolis-SemiBold.woff2) format("woff2");font-weight:600;font-style:normal;font-display:swap;}@font-face{font-family:Lora;src:local("Lora Regular"),local("Lora-Regular"),url(/core/themes/olivero/fonts/lora/lora-v14-latin-regular.woff2) format("woff2");font-weight:400;font-style:normal;font-display:swap;}@font-face{font-family:Lora;src:local("Lora Italic"),local("Lora-Italic"),url(/core/themes/olivero/fonts/lora/lora-v14-latin-italic.woff2) format("woff2");font-weight:400;font-style:italic;font-display:swap;}@font-face{font-family:Lora;src:local("Lora Bold"),local("Lora-Bold"),url(/core/themes/olivero/fonts/lora/lora-v14-latin-700.woff2) format("woff2");font-weight:700;font-style:normal;font-display:swap;}
:root{--font-sans:"m [... snip ..]

As you can see it's preserved.

I think we need actual steps to reproduce here from a clean install of the Drupal core standard profile - i.e. either an example module or theme, or a patch that can be applied to an existing module or theme that shows the issue. It's clear that this case is accounted for in CssOptimizer, there's test coverage, and it also works for me manually testing the steps described in the issue summary.