This issue has been reported in create-react-app and optimize-css-assets-webpack-plugin.
https://github.com/facebook/create-react-app/issues/7980
https://github.com/NMFR/optimize-css-assets-webpack-plugin/issues/118
It appears the bug has been fixed, but the latest released version does not have the fix. I’ve fixed the problem by changing my opacity values from nn% to 0.nn values (e.g. 70% becomes 0.7).