Webpack-compiled CSS file is including Javascript variables and functions

We use a simple application of webpack/mix:

mix.js('resources/js/app.js', 'public/js')
.js('resources/js/cpg.js', 'public/js')
.js('resources/js/editor.js', 'public/js')
.copy('resources/js/ckeditor/dist', 'public/js/editor')
.sass('resources/sass/app.scss', 'public/css')
.sass('resources/sass/cpg.scss', 'public/css')

With webpack.config.js:

module.exports = {
resolve: {
    alias: {
        '@': path.resolve('resources/js'),
    },
},

// https://webpack.js.org/configuration/entry-context/
entry: './resources/js/editor.js',

// https://webpack.js.org/configuration/output/
output: {
    path: path.resolve(__dirname, 'public/js/editor'),
    filename: 'editor.js'
},

module: {
    rules: [
        {
            test: /ckeditor5-[^/\\]+[/\\]theme[/\\]icons[/\\][^/\\]+\.svg$/,

            use: ['raw-loader']
        },
        {
            test: /ckeditor5-[^/\\]+[/\\]theme[/\\].+\.css$/,

            use: [
                {
                    loader: 'style-loader',
                    options: {
                        injectType: 'singletonStyleTag',
                        attributes: {
                            'data-cke': true
                        }
                    }
                },
                {
                    loader: 'postcss-loader',
                    options: styles.getPostCssConfig({
                        themeImporter: {
                            themePath: require.resolve('@ckeditor/ckeditor5-theme-lark')
                        },
                        minify: true
                    })
                }
            ]
        }
    ]
},

// Useful for debugging.
devtool: 'source-map',

// By default webpack logs warnings if the bundle is bigger than 200kb.
performance: { hints: false }
}

Prior to the addition of ckeditor, we had no troubles. But now that ckeditor has been added, the following JS now appears in our compiled cpg.css file:

 function webpackContext(req) {
    var id = webpackContextResolve(req);
    return __webpack_require__(id);
}
function webpackContextResolve(req) {
    if(!__webpack_require__.o(map, req)) {
        var e = new Error("Cannot find module '" + req + "'");
        e.code = 'MODULE_NOT_FOUND';
        throw e;
    }
    return map[req];
}
webpackContext.keys = function webpackContextKeys() {
    return Object.keys(map);
};
webpackContext.resolve = webpackContextResolve;
module.exports = webpackContext;
webpackContext.id = "./node_modules/moment/locale sync recursive ^\\.\\/.*$";

Obviously, this is a problem. JS code does not belong in CSS files, and it trips up our SonarCloud quality gate (for good reason) so we can't deploy anything that's been compiled unless we manually edit the compiled files. Which mostly defeats the purpose of having them.

Further backstory: the section of our project that uses CKEditor was completed by a contractor. So, all of this was merged into our project before we saw that compiled files were improper. The contractor is no longer with the company, so I'm trying to debug on my own and getting nowhere. It seems to be an exceedingly rare bug for Webpack to place JS code in a CSS file.

Progress update: Removing the ckeditor references has no impact. The Webpack just seems to be broken now. Comprehensive node_modules re-install had no effect. It's just broken.

Issue appears to be a copy of https://github.com/laravel-mix/laravel-mix/issues/1976. Upgrading to Mix 6 creates an absolutely absurd amount of problems for my project, so this will just go unresolved.

Followed the instructions here: https://github.com/laravel-mix/laravel-mix/issues/2633#issuecomment-802023077 I was able to resolve the problem.