App Router: root generateMetadata reruns a shared fetch on sibling navigation even when the shared layout UI is preserved

Link to the code that reproduces this issue

https://github.com/Innei/nextjs-generate-metadata-shared-fetch-repro

To Reproduce

1. Clone the reproduction repo.
2. Run npm install.
3. Run npm run build && npm run start.
4. Open http://localhost:3000/alpha.
5. Note the latest server log for [shared-layout] fetch.
6. Navigate to /beta using the in-app link.
7. Observe that [shared-layout] fetch runs again.

Current vs. Expected behavior

Current:

• The shared layout UI is preserved across sibling navigation.
• The root generateMetadata() still re-evaluates during that navigation.
• Because generateMetadata() uses the same shared fetch as the preserved layout, a new [shared-layout] fetch is triggered on the server.
• The client navigation is an _rsc request and includes a refetch marker in next-router-state-tree.

Expected:

• If a shared layout segment is preserved across sibling navigation, root metadata that only depends on that preserved segment could also be reused instead of triggering the shared fetch again.
• If this reuse is intentionally not possible, the docs should state more clearly that preserved layouts do not imply preserved metadata evaluation or preserved shared data reads.

In short: the shared layout UI is preserved, but the shared server data read is not preserved once root generateMetadata() depends on it.

Provide environment information

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 24.6.0: Wed Nov  5 21:33:59 PST 2025; root:xnu-11417.140.69.705.2~1/RELEASE_ARM64_T8112
  Available memory (MB): 24576
  Available CPU cores: 8
Binaries:
  Node: 22.21.0
  npm: 11.6.2
  Yarn: N/A
  pnpm: 10.26.2
Relevant Packages:
  next: 16.2.0-canary.99
  eslint-config-next: N/A
  react: 19.2.4
  react-dom: 19.2.4
  typescript: N/A
Next.js Config:
  output: N/A

Which area(s) are affected?

• Linking and Navigating
• Metadata

Which stage(s) are affected?

• n