Middleware Use Breaks Locale Routing on / Path on Vercel if autodetected locale is not default locale

[neverether]

Run next info (available from version 12.0.8 and up)

/bin/sh: pnpm: command not found

    Operating System:
      Platform: darwin
      Arch: x64
      Version: Darwin Kernel Version 20.5.0: Sat May  8 05:10:33 PDT 2021; root:xnu-7195.121.3~9/RELEASE_X86_64
    Binaries:
      Node: 14.17.5
      npm: 6.14.14
      Yarn: 1.22.5
      pnpm: N/A
    Relevant packages:
      next: 12.0.10
      react: 17.0.2
      react-dom: 17.0.2

What version of Next.js are you using?

12.0.10

What version of Node.js are you using?

14.17.5

What browser are you using?

Chrome

What operating system are you using?

macOS

How are you deploying your application?

Vercel

Describe the Bug

When using automatic locale detection, and the detected locale is NOT the default locale, trying to route to another locale on / results in a failed prefetch error and doesn't complete routing.

In my case my locales are ["ja","en"] and my defaultLocale is ja. My browser language is set to english, and with the automatic locale detection, I'm taken to the /en route. When I try to route to the ja locale in yarn dev or yarn build && yarn start everything works fine. However when I deploy to Vercel I get:

Uncaught (in promise) Error: Failed to preflight request

And routing is cancelled.

My Japanese users with a japanese langauge setting have no issue moving between the two locales. and if I disable the automatic locale detection I will be taken to / with a ja locale as expected, and I can freely route between the two locales.

Expected Behavior

I'm expecting to not get a preflight failure from Vercel, and routing to complete successfully.

This might be related to #34199

To Reproduce

I've created a minimum reproducable repo here using create-next-app and the official example at: https://github.com/vercel/examples/tree/main/edge-functions/basic-auth-password

Repo/Commits on Github: https://github.com/sollective/next-locale-bug-repro/commits/main

I also have it hosted on Vercel here:
https://next-locale-bug-repro.vercel.app/en (Credentials are admin/password)
(Please set your primary language to English in browser)

[maggo]

This happened to me as well, in particular the HEAD request to / is failing.
Removing the middleware file fixes the issue.

[youminkim]

Same issue here. I had to rewrite including locale. Otherwise, 404 error

   const localePrefix = locale === 'default' ? 'en' : locale;
   url.pathname = `/${localePrefix}/_sites/${hostname}`;
   NextResponse.rewrite(url)

[nkzawa]

It should already be fixed by #35911

[github-actions]

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