Link to the code that reproduces this issue

https://github.com/stefanprobst/issue-next-global-not-found

To Reproduce

1. clone repo from https://github.com/stefanprobst/issue-next-global-not-found
2. pnpm install
3. pnpm dev
4. go to http://localhost:3000/xyz
5. expected to see content from global-not-found but instead see default builtin 404 page

Current vs. Expected behavior

global-not-found should be used for all 404

Provide environment information

Operating System:
  Platform: linux
  Arch: x64
  Version: #35~24.04.1-Ubuntu SMP PREEMPT_DYNAMIC Tue Oct 14 13:55:17 UTC 2
  Available memory (MB): 31401
  Available CPU cores: 16
Binaries:
  Node: 24.11.0
  npm: 11.6.1
  Yarn: N/A
  pnpm: 10.20.0
Relevant Packages:
  next: 16.0.2-canary.17 // Latest available version is detected (16.0.2-canary.17).
  eslint-config-next: N/A
  react: 19.2.0
  react-dom: 19.2.0
  typescript: 5.9.3
Next.js Config:
  output: N/A

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

Not Found

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

next dev (local), next start (local)

Additional context

No response