App Router includes unused CSS module in unrelated routes (bundle analyzer + runtime)

Link to the code that reproduces this issue

https://github.com/t18n/next-js-app-router-css-bundle-repro/tree/main

To Reproduce

1. pnpm install
   2.pnpm build:analyzer
2. Open http://localhost:4000/ and check the analyzer output for dashboard and dashboard/users
3. Note Button.module.scss is included and loaded on those routes

Current vs. Expected behavior

• Expected: Button.module.scss only loads on home and not-found routes where Button is used.

• Actual: Button.module.scss is included and loaded for dashboard and dashboard/users, and blocks render.

Provide environment information

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 25.2.0: Tue Nov 18 21:09:40 PST 2025; root:xnu-12377.61.12~1/RELEASE_ARM64_T6000
  Available memory (MB): 32768
  Available CPU cores: 10
Binaries:
  Node: 24.6.0
  npm: 11.5.1
  Yarn: 1.22.22
  pnpm: 10.24.0
Relevant Packages:
  next: 16.1.6 // Latest available version is detected (16.1.6).
  eslint-config-next: N/A
  react: 19.2.3
  react-dom: 19.2.3
  typescript: 5.9.3
Next.js Config:
  output: N/A

Which area(s) are affected? (Select all that apply)

CSS, Turbopack

Which stage(s) are affected? (Select all that apply)

next build (local), Other (Deployed), next start (local)

Additional context

• Repro is local and also impacts production (CSS loads on unrelated routes and blocks render).
• On a larger project with 100+ components this adds ~11s blocking time on Lighthouse/CWV.

Start with the linked reproduction: run pnpm install and pnpm build:analyzer, then inspect the analyzer output for /, /dashboard, and /dashboard/users. Trace why Button.module.scss is included outside the home and not-found routes; done means it loads only where Button is used and no longer blocks unrelated routes.