Link to the code that reproduces this issue

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

To Reproduce

1. clone the repo
2. pnpm install && pnpm build && pnpm start
3. open http://localhost:3000/not-existing-route
4. see the built-in default not-found page being displayed, not the one from /app/(app)/not-found.tsx

Current vs. Expected behavior

the custom root not-found page is not shown for e.g. http://localhost:3000/not-existing-route because it lives in a route group (instead the built-in default 404 page is shown):

└── app
    └── (app)
        ├── layout.tsx
        ├── not-found.tsx
        └── page.tsx

it only works correctly when getting rid of the route group (check out the no-route-group branch)

└── app
    ├── layout.tsx
    ├──	not-found.tsx
    └── page.tsx

Provide environment information

Operating System:
  Platform: linux
  Arch: x64
  Version: #49-Ubuntu SMP PREEMPT_DYNAMIC Mon Nov  4 02:06:24 UTC 2024
  Available memory (MB): 32041
  Available CPU cores: 4
Binaries:
  Node: 22.11.0
  npm: 10.9.0
  Yarn: N/A
  pnpm: 9.14.2
Relevant Packages:
  next: 15.0.4-canary.32 // Latest available version is detected (15.0.4-canary.32).
  eslint-config-next: N/A
  react: 19.0.0-rc-b01722d5-20241114
  react-dom: 19.0.0-rc-b01722d5-20241114
  typescript: 5.7.2
Next.js Config:
  output: N/A

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

Navigation

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

next dev (local), next start (local)

Additional context

No response