Unlisted CSS using preprocessor does not build css file

[PortalCube]

Describe the bug

I encountered a situation where I needed to add a stylesheet dynamically, so I tried using unlisted CSS. I created a .scss as unlisted CSS, but the scss file was not generated as a CSS output.

When adding regular .css as unlisted CSS, it works without any issues, so it seems like there's something wrong related to sass.

Reproduction

unlisted-css-bug.zip

Steps to reproduce

1. npx wxt@latest init
2. npm i
3. npm i sass-embedded -D
4. Create a file entrypoints/test.content.scss
5. Run npm run dev or npm run build

Expected
A file named test.css should be generated in .output/chrome-mv3/content-scripts

Actual
The test.css file does not exist :(

System Info

System:
    OS: Windows 11 10.0.26100
    CPU: (16) x64 AMD Ryzen 9 6900HX with Radeon Graphics
    Memory: 11.68 GB / 27.69 GB
  Binaries:
    Node: 22.13.1 - C:\Program Files\nodejs\node.EXE
    npm: 11.1.0 - C:\Program Files\nodejs\npm.CMD
    pnpm: 10.7.1 - ~\AppData\Local\pnpm\pnpm.CMD
  Browsers:
    Edge: Chromium (132.0.2957.140)
    Internet Explorer: 11.0.26100.1882

Used Package Manager

npm

Validations

• Read the Contributing Guidelines.
• 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.

[PortalCube]

I have submitted a PR to fix this issue. #1590

After further investigation, I found that the same issue not only with Sass but with all CSS preprocessors, so I have updated the title