multi set-cookie merged into one header when cookie name is same

Link to the code that reproduces this issue

https://github.com/yeliex/stackblitz-starters-kfmrzz

To Reproduce

yarn
yarn dev

then open ${host}/api/test, open dev-tools or read terminal output, you would find set-cookie headers.

Current vs. Expected behavior

Expected

set-cookie: session=1713406956812; Path=/; Domain=.example1.com; Secure; HttpOnly; SameSite=none
set-cookie: session=1713406956812; Path=/; Domain=.example2.com; Secure; HttpOnly; SameSite=none

Current

## server response
set-cookie: session=1713406956812; Path=/; Domain=.example1.com; Secure; HttpOnly; SameSite=none, session=1713406956812; Path=/; Domain=.example2.com; Secure; HttpOnly; SameSite=none

## dev-tools response header
Set-Cookie: session=1713407480483; Path=/; Domain=.example2.com; Secure; HttpOnly; SameSite=none

Provide environment information

Operating System:
  Platform: linux
  Arch: x64
  Version: #1 SMP PREEMPT_DYNAMIC Sun Aug  6 20:05:33 UTC 2023
  Available memory (MB): 4102
  Available CPU cores: 2
Binaries:
  Node: 20.12.0
  npm: 10.5.0
  Yarn: 1.22.19
  pnpm: 8.15.6
Relevant Packages:
  next: 14.3.0-canary.9 // Latest available version is detected (14.3.0-canary.9).
  eslint-config-next: N/A
  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)

Not sure

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

next dev (local), Other (Deployed)

Additional context

when set multi cookie with same name, it would merged into one set-cookie header, then when response to client, only the last one remains

Start with the linked reproduction and run yarn followed by yarn dev, then request /api/test and inspect the server and browser response headers. Compare the result with the two expected separate Set-Cookie headers; done means cookies with the same name remain separate and the client receives both values.