Ignored classes using mini-css-extract-plugin or css-loader

Question

I try to use fullcalendar js which includes it's css by js: import { Calendar } from '@fullcalendar/core';

Because of my webpack config (probably wrong somewhere) duplicated class names are ignored for some reason

This is my webconfig's module part:

rules: [
        {
            test: /\.(sass|scss)$/,
            include: path.resolve(__dirname, paths.src.scss.slice(2)),
            use: [
                {
                    loader: MiniCssExtractPlugin.loader
                }, {
                    loader: 'css-loader',
                    options: {
                        url: false
                    }
                }, {
                    loader: 'postcss-loader',
                    options: {
                        postcssOptions: {
                            plugins: [['autoprefixer']]
                        }
                    }
                },
                {
                    loader: 'sass-loader'
                }
            ]
        },
        {
            test: /\.css$/,
            use: [
                MiniCssExtractPlugin.loader,
                'css-loader'
            ]
        }
    ]

Why these classes are ignored?