laravel-mix: .options({processCssUrl: false}) causes .sourceMaps() malfunctioning

• Laravel Mix Version: 5.4.15
• Node Version (node -v): 7.6.0
• NPM Version (npm -v): 4.1.2
• OS: 10.12.3
• laravel-mix: 0.8.8

Description:

When use .sourceMaps() alone in my webpack.mix.js, the source map works fine for Sass compiling. With a fresh installation of Laravel, the generated css file is 1.61 MB. However, if options({processCssUrl: false}) is added, the css file is only 146KB, not minified, and no source map found by the browser. The source map part of the file is as this.

/*# sourceMappingURL=data:application/json;charset=utf-8;base64,eyJ2ZXJzaW9uIjozLCJzb3VyY2VzIjpbXSwibmFtZXMiOltdLCJtYXBwaW5ncyI6IiIsImZpbGUiOiIvY3NzL2FwcC5jc3MiLCJzb3VyY2VSb290IjoiIn0=*/

Steps To Reproduce:

1. laravel new laravel_proj
2. cd laravel_proj
3. npm install
4. open composer.json
5. revise all bin/cross-env.js to dist/bin/cross-env.js (this is a fix found here for another error)
6. revise "laravel-mix": "^0.8.1" to "laravel-mix": "^0.8.8", as suggested by @gudorian to eleminate error message on .options
7. npm i
8. open webpack.mix.js
9. revise to

mix.sass('resources/assets/app/app.scss', 'public/css')
   .options({
      processCssUrls: false
   });
mix.sourceMaps()

1. npm run dev
2. result

            Asset      Size  Chunks                    Chunk Names
     /css/app.css    146 kB       0  [emitted]         /js/app