Sitemap generation does not follow file location specification

Link to the code that reproduces this issue

https://github.com/dretsa/next-sitemap-spec

To Reproduce

1. Generate multiple sitemaps as per documentation
2. Check paths generated and see they do not follow the specification

Example - /app/product/sitemap.ts generates - from docs example:

• /product/sitemap/0.xml
• /product/sitemap/1.xml
• /product/sitemap/2.xml
• /product/sitemap/3.xml

According to the specification, these sitemaps can only contain links starting with /product/sitemap/....., as specified in the "Sitemap file location" section.

Current vs. Expected behavior

I expect the generated sitemaps in a route app/product/sitemap.ts to allow me link /product/... pages. Currently they can only provide URLs starting with /product/sitemap/...

Provide environment information

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 24.3.0: Thu Jan  2 20:24:22 PST 2025; root:xnu-11215.81.4~3/RELEASE_ARM64_T6041
  Available memory (MB): 49152
  Available CPU cores: 14
Binaries:
  Node: 22.2.0
  npm: 10.7.0
  Yarn: 4.6.0
  pnpm: N/A
Relevant Packages:
  next: 15.1.7
  eslint-config-next: 15.1.7
  react: 19.0.0
  react-dom: 19.0.0
  typescript: 5.7.3
Next.js Config:
  output: standalone

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

Metadata, Route Handlers

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

next build (local), next start (local), next dev (local), Vercel (Deployed), Other (Deployed)

Additional context

No response

Start with the linked next-sitemap-spec reproduction and the app/product/sitemap.ts example, then compare the generated paths with the Sitemap file location specification in the linked Next.js documentation. Trace the metadata sitemap generation behavior involved in multiple sitemaps. Done means generated URLs can reference /product/... pages while remaining valid under the specification.