Loading `@import`-ed CSS files breaks Webpack Hot Reloading

Start with the linked reproduction repository: run yarn install and yarn run start, then reproduce the error-recovery sequence using main.css and its @import-ed imported.css. Trace why correcting an invalid edit in imported.css is not detected, and consider the issue done when hot reloading rebuilds it successfully without restarting webpack.

Bug report

Actual Behavior

If you make an edit to an @import-ed CSS file that results in an error (e.g. invalid CSS), webpack hot reloader no longer detects further changes in that file - even if you correct or undo the edit (CTRL-Z). You need to restart webpack for it to start detecting the file again.

This is only in the specific scenario of a breaking change. Any valid change continues to be detected by hot reloading.

This is also specific to only @import-ed files. Any change to a top-level file that is not imported by another CSS file continues work with hot reloading.

Expected Behavior

Webpack Hot Reloader should continue detecting updates to the previously edited file. If the error is corrected, it should recompile and emit the successful file.

How Do We Reproduce?

Here is a repository to reproduce it: https://github.com/abhchand/webpack-postcss-hmr-issue-reproduction. This repo uses postcss to run Tailwind CSS.

1. Clone the repo
2. Run yarn install, then yarn run start
3. Open http://localhost:3035/

You'll notice there are 2 CSS files - main.css and imported.css. The second one is @import-ed by the first.

You can reproduce the following scenarios:

The only way to fix the last scenario is to restart webpack.

Please paste the results of npx webpack info here

$ npx webpack info

  System:
    OS: Linux 5.15 Ubuntu 20.04.6 LTS (Focal Fossa)
    CPU: (12) x64 Intel(R) Core(TM) i7-10750H CPU @ 2.60GHz
    Memory: 7.60 GB / 15.36 GB
  Binaries:
    Node: 18.20.7 - ~/.nvm/versions/node/v18.20.7/bin/node
    Yarn: 1.22.22 - ~/.yarn/bin/yarn
    npm: 10.8.2 - ~/.nvm/versions/node/v18.20.7/bin/npm
  Browsers:
    Chrome: 117.0.5938.132
  Packages:
    babel-loader: ^8.1.0 => 8.4.1
    css-loader: ^4.2.0 => 4.3.0
    html-webpack-plugin: ^5.5.0 => 5.6.3
    postcss-loader: ^7.0.1 => 7.3.4
    webpack: ^5.74.0 => 5.98.0
    webpack-cli: 4.10.0 => 4.10.0
    webpack-dev-server: ^4.11.1 => 4.15.2