next.js 15 after() can not get response.body

[kin-for-test]

Link to the code that reproduces this issue

https://codesandbox.io/p/devbox/delicate-cloud-h9jx3h

To Reproduce

in middleware.ts , after() can not catch response.body

Current vs. Expected behavior

Current : null
Expected :"{a:123}"

Provide environment information

Operating System:
  Platform: win32
  Arch: x64
  Version: Windows 10 Pro
  Available memory (MB): 16220
  Available CPU cores: 12
Binaries:
  Node: 21.0.0
  npm: N/A
  Yarn: N/A
  pnpm: N/A
Relevant Packages:
  next: 15.0.0-canary.54
  eslint-config-next: 14.2.4
  react: 19.0.0-rc-e02baf6c92-20240627
  react-dom: 19.0.0-rc-e02baf6c92-20240627
  typescript: 5.5.2
Next.js Config:
  output: N/A

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

Middleware

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

next dev (local), next start (local)

Additional context

No response