The solution was to import all the required css into one main css file, then use that inside the mix chain:

in app.css we import all of our css files

@import 'resources/css/plugin-1.css';
@import 'resources/css/plugin-2.css';


inside webpack.mix.js we use resource/app.css as an input

mix.styles([
'resources/app.css',
], output);