Link to the code that reproduces this issue

https://github.com/amannn/nextjs-bugrepro-sitemap

To Reproduce

1. Run the app
2. Visit /sitemap.xml

Current vs. Expected behavior

For app/sitemap.ts to be served at /sitemap.xml instead of being served at /sitemap. I suspect this started happening after this pull request was merged: #65507

Someone else has experienced the same problem and raised it in that pull request.

Provide environment information

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 23.5.0: Wed May  1 20:17:33 PDT 2024; root:xnu-10063.121.3~5/RELEASE_ARM64_T6031
  Available memory (MB): 65536
  Available CPU cores: 16
Binaries:
  Node: 20.5.1
  npm: 9.8.0
  Yarn: N/A
  pnpm: 9.1.0
Relevant Packages:
  next: 15.0.0-rc.0 // Latest available version is detected (15.0.0-rc.0).
  eslint-config-next: N/A
  react: 19.0.0-rc-f994737d14-20240522
  react-dom: 19.0.0-rc-f994737d14-20240522
  typescript: 5.4.5
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), next build (local), next start (local), Vercel (Deployed)

Additional context

This is seems to be working with NextJS 14.2.3. The problem is happening on NextJS 15.0.0 RC 0.