CSS not included in lazy-loading legacy files in Vite 3.0.0-alpha.1

[X-Y]

Describe the bug

When using Vite 3.0.0-alpha.1, Legacy builds of lazy-loaded components don't have css included anymore. @vitejs/plugin-legacy does not seem to be the issue, I have tried both 1.8.2 and 2.0.0-alpha.0, neither would include css. When building with Vite 2.9.9, the css is in.

I include css through a simple import: import './DateSelectLazy.css';

You can see my minimum reproduction code in https://github.com/X-Y/vite-base-path-ie11-problem-demo/tree/cssLostIn3

If you use instead https://github.com/X-Y/vite-base-path-ie11-problem-demo/tree/cssLostIn3-before3 and refresh the dependencies, the builds would have css included.

Reproduction

https://x-y.github.io/vite-base-path-ie11-problem-demo

If you open this page in ie11, and click the button, you can see the revealed text is not red.
If you look at the legacy file: https://x-y.github.io/vite-base-path-ie11-problem-demo/assets/DateSelectLazy-legacy.df3c73b5.js You would see that no css is included.

System Info

System:
    OS: macOS 11.4
    CPU: (4) x64 Intel(R) Core(TM) i7-7660U CPU @ 2.50GHz
    Memory: 16.93 MB / 8.00 GB
    Shell: 5.8 - /bin/zsh
  Binaries:
    Node: 16.13.1 - ~/.nvm/versions/node/v16.13.1/bin/node
    Yarn: 1.22.17 - ~/.nvm/versions/node/v16.13.1/bin/yarn
    npm: 8.1.2 - ~/.nvm/versions/node/v16.13.1/bin/npm
    Watchman: 4.9.0 - /usr/local/bin/watchman
  Browsers:
    Chrome: 101.0.4951.64
    Firefox: 100.0
    Safari: 14.1.1
  npmPackages:
    @vitejs/plugin-legacy: 1.8.2 => 1.8.2 
    @vitejs/plugin-react: ^1.3.0 => 1.3.2 
    vite: 3.0.0-alpha.1 => 3.0.0-alpha.1

Used Package Manager

yarn

Logs

No response

Validations

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• Read the docs.
• Check that there isn't already an issue that reports the same bug to avoid creating a duplicate.
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• The provided reproduction is a minimal reproducible example of the bug.

[sapphi-red]

It does not happen with 80d113b and it happens with 780b4f5.
So #5902 is affecting this.