Not-found broken with multiple root layouts

### Link to the code that reproduces this issue https://codesandbox.io/p/devbox/blissful-fog-nflrh6 ### To Reproduce 1. Open sandbox and click `not found page` 2. It will redirect to the generic nextjs 404 page ### Current vs. Expected behavior Generic 404 Nextjs error page is shown when I navigate to any unknow urls. I expect to be redirected to my custom 404 UI handler ### Verify canary release - [X] I verified that the issue exists in the latest Next.js canary release ### Provide environment information ```bash Operating System: Platform: darwin Arch: x64 Version: Darwin Kernel Version 22.6.0: Wed Jul 5 22:21:56 PDT 2023; root:xnu-8796.141.3~6/RELEASE_X86_64 Binaries: Node: 20.9.0 npm: 10.1.0 Yarn: N/A pnpm: 8.9.0 Relevant Packages: next: 14.0.3 eslint-config-next: 13.5.6 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, Routing (next/router, next/navigation, next/link) ### Additional context > Good to know: In addition to catching expected notFound() errors, the root app/not-found.js file also handles any unmatched URLs for your whole application. This means users that visit a URL that is not handled by your app will be shown the UI exported by the app/not-found.js file. https://nextjs.org/docs/app/api-reference/file-conventions/not-found --- According to the docs in order to be able to handle any unmatched URLs in the application we are bound to the fixed path for the not-found UI that must be placed at the root of the app -> `app/not-found.js` This breaks not-found custom UI handling as per provided example. I've 2 root layout groups https://nextjs.org/docs/app/building-your-application/routing/route-groups#creating-multiple-root-layouts 1. Auth group (which is publicly accessible) 2. Main group (which is accessible if user is authenticated and basically contains the whole app routing) With this setup and not having app root not-f…