Link to the code that reproduces this issue

https://github.com/alyustik/nextjs-notfound-regression

To Reproduce

1. Start an application in any mode (e.g. npm run dev)
2. Navigate to non-existing page (e.g. http://localhost:3000/non-existing-page)

Current vs. Expected behavior

Current: global metadata, that was defined in root layout is missing on the non-existing page
Expected: global metadata preserves on non-existing page

Provide environment information

Operating System:
  Platform: win32
  Arch: x64
  Version: Windows 11 Pro
  Available memory (MB): 64662
  Available CPU cores: 16
Binaries:
  Node: 22.5.1
  npm: 10.8.2
  Yarn: N/A
  pnpm: 10.5.2
Relevant Packages:
  next: 15.3.0-canary.22 // Latest available version is detected (15.3.0-canary.22).
  eslint-config-next: N/A
  react: 19.0.0
  react-dom: 19.0.0
  typescript: 5.8.2
Next.js Config:
  output: N/A

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

Not Found, Metadata

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

next dev (local), next build (local), next start (local)

Additional context

If it helps, that only happens on pages, where notFound() was invoked.
So if I would create directories like

• app/about/page.tsx
• app/articles/[slug]/page.tsx

And then I navigate to /non-existing-page - meta would work.
But if I navigate to /articles/non-existing-page - meta wouldn't work.

Happens with both generateMetadata() and metadata object.

I also tried to add metadata on not-found.tsx but it was ignored.

Issue can be reproduced starting from 15.2.0