Hot module reloading for CSS is not working in the mini-css-extract-plugin.
$ pnpm install
$ pnpm run devModify src/index.css.
Style in src/async.css is covered by src/index.css.
After reproduction, If you modify src/async.css, the order of styles is back to normal. At this time, you can modify src/index.css, and you can see the problem is gone.
Since async.js is imported below index.css, the style of async.js(i.e async.css) should be placed after index.css.