regression introduced between 13.4.13 - 13.4.15 - appdir + custom server = assetPath issue

[morriq]

Link to the code that reproduces this issue

https://github.com/morriq/next-cdn-issue/

To Reproduce

1. clone repository https://github.com/morriq/next-cdn-issue/
2. go to ./apps/app-dir-custom-server
3. pnpm i && pnpm build && pnpm start
4. in browser you should see some assets loaded, some not

Current vs. Expected behavior

based on reproduction I expect all assets are not loaded. assets are served on different server.

Provide environment information

Operating System:
  Platform: darwin
  Arch: x64
  Version: Darwin Kernel Version 23.3.0: Wed Dec 20 21:30:44 PST 2023; root:xnu-10002.81.5~7/RELEASE_ARM64_T6000
Binaries:
  Node: 18.18.2
  npm: 9.8.1
  Yarn: 1.22.19
  pnpm: 8.6.7
Relevant Packages:
  next: 14.1.0
  eslint-config-next: N/A
  react: 18.0.0
  react-dom: 18.0.0
  typescript: N/A
Next.js Config:
  output: N/A

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

App Router

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

next start (local), Other (Deployed)

Additional context

related issue #51223

[github-actions]

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