Duplicated styles with suspended components in multiple routes since v7.9.5

[gu-stav]

Reproduction

https://stackblitz.com/edit/github-sctnoygg-bjyrxehw?initialpath=slug

System Info

System:
    OS: macOS 26.2
    CPU: (12) arm64 Apple M4 Pro
    Memory: 935.80 MB / 48.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 25.2.1 - /opt/homebrew/bin/node
    npm: 11.6.2 - /opt/homebrew/bin/npm
    pnpm: 10.27.0 - /opt/homebrew/bin/pnpm
  Browsers:
    Chrome: 144.0.7559.97
    Chrome Canary: 146.0.7652.0
    Firefox Nightly: 149.0a1
    Safari: 26.2

Used Package Manager

npm

Expected Behavior

I expect the CSS file WithStyles.css only to be downloaded once:

Actual Behavior

The CSS file is downloaded twice.

The behavior seems to be correct as soon as I remove the component import in routes/$slug/layout.tsx or remove the index route in routes.ts.

The regression started with react-router@7.9.5.

[thomaswelton]

Thanks for opening this with a reproduction @gu-stav
I've been debugging this issue assuming it was an issue with something in our application, but we're seeing the same issue you are. For us this then causes CSS specificity issues as that CSS is now in a different order than it should be pre v7.9.5