Repository: next.js
Issue: Global not-found not available in multiple root layouts
Labels: bug

### Link to the code that reproduces this issue or a replay of the bug

https://github.com/devjiwonchoi/multi-root-layout-not-found

### To Reproduce

### [CodeSandBox Repro Link](https://codesandbox.io/p/github/devjiwonchoi/multi-root-layout-not-found/main?layout=%257B%2522sidebarPanel%2522%253A%2522EXPLORER%2522%252C%2522rootPanelGroup%2522%253A%257B%2522direction%2522%253A%2522horizontal%2522%252C%2522contentType%2522%253A%2522UNKNOWN%2522%252C%2522type%2522%253A%2522PANEL_GROUP%2522%252C%2522id%2522%253A%2522ROOT_LAYOUT%2522%252C%2522panels%2522%253A%255B%257B%2522type%2522%253A%2522PANEL_GROUP%2522%252C%2522contentType%2522%253A%2522UNKNOWN%2522%252C%2522direction%2522%253A%2522vertical%2522%252C%2522id%2522%253A%2522clmd2qfhw000j3b6f0zxp55jd%2522%252C%2522sizes%2522%253A%255B70%252C30%255D%252C%2522panels%2522%253A%255B%257B%2522type%2522%253A%2522PANEL_GROUP%2522%252C%2522contentType%2522%253A%2522EDITOR%2522%252C%2522direction%2522%253A%2522horizontal%2522%252C%2522id%2522%253A%2522EDITOR%2522%252C%2522panels%2522%253A%255B%257B%2522type%2522%253A%2522PANEL%2522%252C%2522contentType%2522%253A%2522EDITOR%2522%252C%2522id%2522%253A%2522clmd2qfhw000e3b6fnqsxx5to%2522%257D%255D%252C%2522sizes%2522%253A%255B100%255D%257D%252C%257B%2522type%2522%253A%2522PANEL_GROUP%2522%252C%2522contentType%2522%253A%2522SHELLS%2522%252C%2522direction%2522%253A%2522horizontal%2522%252C%2522id%2522%253A%2522SHELLS%2522%252C%2522panels%2522%253A%255B%257B%2522type%2522%253A%2522PANEL%2522%252C%2522contentType%2522%253A%2522SHELLS%2522%252C%2522id%2522%253A%2522clmd2qfhw000i3b6fr0m08gcp%2522%257D%255D%252C%2522sizes%2522%253A%255B100%255D%257D%255D%257D%252C%257B%2522type%2522%253A%2522PANEL_GROUP%2522%252C%2522contentType%2522%253A%2522DEVTOOLS%2522%252C%2522direction%2522%253A%2522vertical%2522%252C%2522id%2522%253A%2522DEVTOOLS%2522%252C%2522panels%2522%253A%255B%257B%2522type%2522%253A%2522PANEL%2522%252C%2522contentType%2522%253A%2522DEVTOOLS%2522%252C%2522id%2522%253A%2522clmd2qfhw000g3b6fzlkqt7di%2522%257D%255D%252C%2522sizes%2522%253A%255B100%255D%257D%255D%252C%2522sizes%2522%253A%255B50%252C50%255D%257D%252C%2522tabbedPanels%2522%253A%257B%2522clmd2qfhw000e3b6fnqsxx5to%2522%253A%257B%2522tabs%2522%253A%255B%257B%2522id%2522%253A%2522clmd2qfhw000d3b6fe0wct87c%2522%252C%2522mode%2522%253A%2522permanent%2522%252C%2522type%2522%253A%2522FILE%2522%252C%2522filepath%2522%253A%2522%252FREADME.md%2522%257D%255D%252C%2522id%2522%253A%2522clmd2qfhw000e3b6fnqsxx5to%2522%252C%2522activeTabId%2522%253A%2522clmd2qfhw000d3b6fe0wct87c%2522%257D%252C%2522clmd2qfhw000g3b6fzlkqt7di%2522%253A%257B%2522id%2522%253A%2522clmd2qfhw000g3b6fzlkqt7di%2522%252C%2522activeTabId%2522%253A%2522clmd2qvm000ne3b6fw8cbg3yz%2522%252C%2522tabs%2522%253A%255B%257B%2522type%2522%253A%2522TASK_PORT%2522%252C%2522taskId%2522%253A%2522dev%2522%252C%2522port%2522%253A3000%252C%2522id%2522%253A%2522clmd2qvm000ne3b6fw8cbg3yz%2522%252C%2522mode%2522%253A%2522permanent%2522%252C%2522path%2522%253A%2522%252F404%2522%257D%255D%257D%252C%2522clmd2qfhw000i3b6fr0m08gcp%2522%253A%257B%2522id%2522%253A%2522clmd2qfhw000i3b6fr0m08gcp%2522%252C%2522activeTabId%2522%253A%2522clmd2qx2p00rc3b6fgjp8py4p%2522%252C%2522tabs%2522%253A%255B%257B%2522id%2522%253A%2522clmd2qfhw000h3b6f2vbvb6pi%2522%252C%2522mode%2522%253A%2522permanent%2522%252C%2522type%2522%253A%2522TERMINAL%2522%252C%2522shellId%2522%253A%2522clmd2qfou000se5jh3ojyaatf%2522%257D%252C%257B%2522type%2522%253A%2522TASK_LOG%2522%252C%2522taskId%2522%253A%2522dev%2522%252C%2522id%2522%253A%2522clmd2qtii00j53b6f2r8yoxuq%2522%252C%2522mode%2522%253A%2522permanent%2522%257D%252C%257B%2522id%2522%253A%2522clmd2qx2p00rc3b6fgjp8py4p%2522%252C%2522mode%2522%253A%2522permanent%2522%252C%2522type%2522%253A%2522TERMINAL%2522%252C%2522shellId%2522%253A%2522clmd2qx7i000we5jhc1noagxc%2522%257D%255D%257D%257D%252C%2522showDevtools%2522%253Atrue%252C%2522showShells%2522%253Atrue%252C%2522showSidebar%2522%253Atrue%252C%2522sidebarPanelSize%2522%253A15%257D)

1. Go to the repro link
2. Go to `/404`

### Current vs. Expected behavior

### Current

The multi-root layout group routes cannot have a global not-found since they don't have `app/layout`.

> src/app/not-found.tsx
not-found.tsx doesn't have a root layout. To fix this error, make sure every page has a root layout.

I'm trying to clarify how to handle the unmatched URLs if global not-found is not allowed in group routes with multi-root layouts.

### Expected

IMO the expected behavior for `/404` is displaying the not-found inside the group which has app/(group)/page.tsx.

### Verify canary release

- [X] I verified that the issue exists in the latest Next.js canary release

### Provide environment information

```bash
Operating System:
  Platform: linux
  Arch: x64
  Version: #1 SMP PREEMPT_DYNAMIC Sun Aug  6 20:05:33 UTC 2023
Binaries:
  Node: 16.17.0
  npm: 8.15.0
  Yarn: 1.22.19
  pnpm: 7.1.0
Relevant Packages:
  next: 13.4.20-canary.23
  eslint-config-next: 13.4.20-canary.23
  react: 18.2.0
  react-dom: 18.2.0
  typescript: 5.2.2
Next.js Config:
  output: N/A
```


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

App Router

### Additional context

This issue extends #54980