webpack4.x打包时

Question

webpack4.x打包时 在a.js中require b.js时，把b.js中引入的css打包到了a.js引入的css中怎么回事？
以下是webpack.config.js配置
请各位大佬指点

const path = require('path');
const htmlWebpackPlugin = require('html-webpack-plugin');
const cleanWebpackPlugin = require('clean-webpack-plugin');
const MiniCssExtractPlugin = require("mini-css-extract-plugin");//提取css到单独文件的插件
const ExtractTextPlugin = require("extract-text-webpack-plugin");


var getHtmlConfig = function (filename,title) {
    return {
        template:'./src/' + filename + '/'+ filename + '.html',
        filename: filename + '.html',
        title:title,
        hash:true,
        chunks:[filename]
    }
}
module.exports = {
    entry: {
        index:'./src/index/index.js',
        test:'./src/test/test.js'
    },
    output: {
        filename: './js/[name].bundle.js',
        path: path.resolve(__dirname,'dist'),
    },
    devServer:{
        contentBase: './dist'
    },
    plugins: [
        new htmlWebpackPlugin(getHtmlConfig("index",'index')),
        new htmlWebpackPlugin(getHtmlConfig("test",'test')),
        new cleanWebpackPlugin(['dist'])
    ],
    module:{
        rules:[
            {
                test:/\.css$/,
                use:ExtractTextPlugin.extract({
                    fallback: "style-loader",
                    use: "css-loader"
                })
            },
            {
                test:/\.(png|svg|jpg|gif)$/,
                use:[
                    'file-loader'
                ]
            },
            {
                test: /\.(woff|woff2|eot|ttf|otf)$/,
                use:[
                    'file-loader'
                ]
            }
        ]
    }