mix.version() changes hashes for changed and unchanged CSS files

I have the following code in my webpack.mix.js file

mix.js('resources/assets/js/app.js', 'public/js') .sass('resources/assets/sass/app.scss', 'public/css') .sass('resources/assets/sass/test.scss', 'public/css');

mix.version();

app.scss and test.scss are independent but everytime I run npm run dev both hashes in the generated css files are changed to the same new hash, regardless of which one has changed. If I only change app.scss, the hash of test.css changes too. Shouldn't it only change the hash for the updated css file?

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