Next.js routes respond with 200 to unsupported HTTP methods

Link to the code that reproduces this issue

https://codesandbox.io/p/sandbox/github/vercel/next.js/tree/canary/examples/reproduction-template

To Reproduce

1. Open codesandbox
2. Copy Preview URL
3. Using your tool of choice, make any non GET request (POST, PATCH, DELETE, etc)

Current vs. Expected behavior

Current:

• 200 response, with HTML document matching the GET route
• Client side error once JS loads

Application error: a client-side exception has occurred while loading (see the browser console for more information).

Expected:
405 - Method not allowed

Provide environment information

Operating System:
      Platform: darwin
      Arch: arm64
      Version: Darwin Kernel Version 23.0.0: Fri Sep 15 14:41:43 PDT 2023; root:xnu-10002.1.13~1/RELEASE_ARM64_T6000
    Binaries:
      Node: 20.9.0
      npm: 10.1.0
      Yarn: 3.5.0
      pnpm: 10.5.2
    Relevant Packages:
      next: 13.4.19
      eslint-config-next: 15.3.0
      react: 18.3.1
      react-dom: 18.3.1
      typescript: 5.8.3
    Next.js Config:
      output: N/A

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

Not sure

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

Vercel (Deployed), next start (local), next dev (local), next build (local), Other (Deployed)

Additional context

I've tested this both self hosted and on Vercel, with 200's on each. The next.js website returns a 405 as expected.

Start with the linked CodeSandbox reproduction template and send POST, PATCH, and DELETE requests to the copied Preview URL. Compare behavior in next dev, next start, next build, and deployed environments; done means unsupported methods consistently return 405 instead of the GET HTML response.