mix creates css under js folder

Laravel 8 and mix 6.

I discovered mix is generating a js/vendor.css file

Why ?

my larave-mix is

const mix = require('laravel-mix');
const webpack = require('webpack');
const env = process.env.NODE_ENV || 'development';
const config = require('./config');

const extracted_to_vendor = [
    "axios",
    "bootstrap",
    "bootstrap-datepicker",
    "bootstrap-select",
    "bootstrap-toggle",
    "dropzone",
    "es6-promise",
    "fontawesome",
    "js-cookie",
    "jwt-decode",
    "lodash",
    "select2",
    "summernote",
    "sweetalert2",
    "toastr",
    "v-select2-component",
    "vue-carousel",
    "vue-progressbar",
    "vue-snotify",
    "vue2-filters",
    "vuejs-dialog",
    "vuex",
    '@fullcalendar/core',
    '@fullcalendar/daygrid',
    '@fullcalendar/bootstrap',
];


const uglifyOptions = {
    development: {},
    production: {
        uglifyOptions: {
            compress: {
                warnings: false,
                conditionals: true,
                unused: true,
                comparisons: true,
                sequences: true,
                dead_code: true,
                evaluate: true,
                if_return: true,
                join_vars: true,
            },
            output: {
                comments: false
            }
        }
    }
};

mix
    .extract(['jquery', './resources/assets/js/jquery-global.js'], 'public/js/jquery.js')
    .extract(extracted_to_vendor, 'public/js/vendor.js')

mix.js('resources/assets/js/frontend.js', 'public/js').vue({
    extractStyles: true,
    globalStyles: false
})

mix.sass('resources/assets/sass/frontend/frontend.scss', 'public/css')

mix.js('resources/assets/js/backend.js', 'public/js')
mix.sass('resources/assets/sass/backend/backend.scss', 'public/css')


mix.browserSync(config.browserSyncUrl)
    .options({
        terser: {
            extractComments: false,
        },
        processCssUrls: false,
        uglify: uglifyOptions[env]
    });
mix.disableSuccessNotifications();

mix.webpackConfig({
    plugins: [
        new webpack.ProvidePlugin({
            $: "jquery",
            jQuery: "jquery",
            Vue: "vue"
        })
    ],
    resolve: {
        fallback: {
            "path": require.resolve("path-browserify")
        }
    }
});

mix.version();