laravel-mix produces empty (0 bytes) .css files WITHOUT dynamic imports and .extract

I'm getting empty .css files without using .extract and import() When I'm running npm run dev

My webpack.mix.js:

let mix = require('laravel-mix');
mix.js('resources/assets/js/app.js', 'public/js').sourceMaps()
    .js('resources/assets/js/admin.js', 'public/js').sourceMaps()
    .sass('resources/assets/sass/app.scss', 'public/css')
    .sass('resources/assets/sass/admin.scss', 'public/css');

mix.copyDirectory('resources/assets/img', 'public/img');

The strange thing here is that if I remove one of .js mixes it works. For example this works:

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

this also works:

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

But I need to generate admin.js (or app.js) after that. Which is not very handy. Any ideas what I'm doing wrong?

My environment is:

• Laravel Mix Version: 5.0.0
• Node Version: 11.0.0
• NPM Version: 6.10.3
• OS: macOS High Siera / Ubuntu 18.04 LTS

I'm sure that I have no dynamic imports, because I'm upgrading from laravel-mix v.3. But also I checked my code. And if I have dynamic imports, none of my examples will work.

EDIT: I found that the problem is in my admin.js file which imports:

import router from './router'

and in /router/index.js there is another import:

import routes from './routes.js'

and the problem seеms to be in routes.js file which looks like that:

const routes = [
    {
        path: '/',
        component: resolve => require(['../views/admin/dashboard/dashboard.vue'], resolve),
        meta: {
            title: "Dashboard",
        }
    },
];
export default routes

But, I can't still explain where is the dynamic import here?!?!? Or how this works:

mix.js('resources/assets/js/admin.js', 'public/js').sourceMaps()
    .sass('resources/assets/sass/admin.scss', 'public/css');