[FLUSH CHUNKS] warning with TSX, but not with JS

[scummbar]

Hello, I get this warning:
[FLUSH CHUNKS]: Unable to find styles/localhost-theme-css in Webpack chunks. Please check usage of Babel plugin.

Following code causes the warning (for setup of react-universal-component):

export default (props) => {
  if (props.site !== undefined) {
    import(`../styles/${props.site}/theme.css`);
  }

Above code is in Routes.tsx, whole file looks like:

import React from "react"
import universal from "react-universal-component"
import { Switch } from "react-router"

const determineHowToLoad = ({ page }) => {
  if (typeof page !== 'string') {
    return page();
  } else {
    return import(`./${page}`);
  }
}

const UniversalComponent = universal(determineHowToLoad, {
  loadingTransition: false
})

export default (props) => {
  if (props.site !== undefined) {
    import(`../styles/${props.site}/theme.css`);
  }

  return (
    <div>
      Test
    </div>
  )
}

However, this happens only if when the filename is Routes.tsx. If I change to Routes.js, no warning occurs. Even with the warning and filename being Routes.tsx, all the things looks working well but only warning occurs in console terminal.

My webpack setting:
1. webpack.dev-client.js:

optimization: {
    splitChunks: {
      chunks: "initial",
      cacheGroups: {
        vendors: {
          test: /[\\/]node_modules[\\/]/,
          name: "vendor"
        }
      }
    }
  },
  devtool: "source-map",
  module: {
    rules: [
      {
        test: /\.js$/,
        exclude: /node_modules/,
        use: [
          {
            loader: "babel-loader"
          }
        ]
      },
      {
        test: /\.(ts|tsx)$/,
        exclude: /node_modules/,
        use: [
          {
            loader: "ts-loader"
          }
        ]
      },
      {
        test: /\.(js|jsx)$/,
        use: 'react-hot-loader/webpack',
        include: /node_modules/
      },
      {
        test: /\.css$/,
        use: [
          ExtractCssChunks.loader, "css-loader",
        ]
      },
....
resolve: {
    extensions: [".ts", ".tsx", ".js", ".css"]
  },

2. webpack.dev-server.js:

devtool: "inline-source-map",
  module: {
    rules: [
      {
        test: /\.js$/,
        exclude: /node_modules/,
        use: [
          {
            loader: "babel-loader"
          }
        ]
      },
      {
        test: /\.(ts|tsx)$/,
        exclude: /node_modules/,
        use: [
          {
            loader: "ts-loader"
          }
        ]
      },
      {
        test: /\.css$/,
        use: [
          ExtractCssChunks.loader, "css-loader"
        ]
      },
....
 resolve: {
    extensions: [".ts", ".tsx", ".js", ".css"]
  },

How can I solve without using js?