Unexpected middleware response headers present on headers() in app router

[gaspar09]

Verify canary release

• I verified that the issue exists in the latest Next.js canary release

Provide environment information

    Operating System:
      Platform: darwin
      Arch: arm64
      Version: Darwin Kernel Version 22.5.0: Thu Jun  8 22:22:23 PDT 2023; root:xnu-8796.121.3~7/RELEASE_ARM64_T6020
    Binaries:
      Node: 16.19.1
      npm: 8.19.3
      Yarn: 1.22.19
      pnpm: 7.30.3
    Relevant Packages:
      next: 13.4.10-canary.3
      eslint-config-next: N/A
      react: 18.2.0
      react-dom: 18.2.0
      typescript: 5.1.6
    Next.js Config:
      output: N/A

Which area(s) of Next.js are affected? (leave empty if unsure)

App Router, Middleware / Edge (API routes, runtime)

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

https://github.com/gaspar09/set-set-headers-middleware-nextjs

To Reproduce

visit https://set-set-headers-middleware-nextjs.vercel.app/header?h=2
See the headers:

x-vercel-header-request = 2
x-vercel-header-response = 2

Describe the Bug

A response header x-vercel-header-response is set in middleware.
That header is available on an app router page via import { headers } from "next/headers";

Expected Behavior

Expected that x-vercel-header-response is not present on headers()

Which browser are you using? (if relevant)

Chrome

How are you deploying your application? (if relevant)

Vercel

_NEXT-1435

[nextjs-bot]

This issue has been automatically marked as stale due to two years of inactivity. It will be closed in 7 days unless there’s further input. If you believe this issue is still relevant, please leave a comment or provide updated details. Thank you.

[nextjs-bot]

This issue has been automatically closed due to two years of inactivity. If you’re still experiencing a similar problem or have additional details to share, please open a new issue following our current issue template. Your updated report helps us investigate and address concerns more efficiently. Thank you for your understanding!