The menu_icons.css is not included in the aggregated CSS when CSS aggreation is activated and site mode is in production mode (drupal site:mode prod) using drupal console. (when CSS aggreation is off and site mode is in production mode, css is included). I tested this in version 2.1 and latest dev.
My workaround for version 2.1:
Add
/sites/default/files/css/menu_icons.css: {}
to the theme libraries.yml
Needs review
2.0
Code
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
Same problem also with 3.0 branch. I managed to solve by removing file prefixing, not sure if it's the right way, but seems to work.
I updated the patch with a couple of improvements. I agree that the CSS file does not need a prefix.