Alternates media in multiple generated sitemap

Link to the code that reproduces this issue

https://codesandbox.io/p/devbox/practical-raman-plx3jf

To Reproduce

1. Start the app (next dev)
2. Go to /pathname/sitemap/0.xml
3. See that we don't have the tag alternate for media

Current vs. Expected behavior

Expected xml

<?xml version="1.0" encoding="UTF-8"?>
<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9" xmlns:xhtml="http://www.w3.org/1999/xhtml">
<url>
<loc>https://example.com/pathname/1</loc>
<xhtml:link rel="alternate" media="only screen and (max-width: 640px)" href="https://m.example.com/pathname/1" />
</url>
</urlset>

Actual xml

<?xml version="1.0" encoding="UTF-8"?>
<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9" xmlns:xhtml="http://www.w3.org/1999/xhtml">
<url>
<loc>https://example.com/pathname/1</loc>
</url>
</urlset>

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: 20.9.0
  npm: 9.8.1
  Yarn: 1.22.19
  pnpm: 8.10.2
Relevant Packages:
  next: 16.0.0-canary.15 // Latest available version is detected (16.0.0-canary.15).
  eslint-config-next: N/A
  react: 19.2.0
  react-dom: 19.2.0
  typescript: 5.3.3
Next.js Config:
  output: N/A

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

Webpack

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

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

Additional context

No response

Start with the linked CodeSandbox reproduction and request /pathname/sitemap/0.xml under next dev, then compare the generated XML with the expected output in the issue. Trace the multiple sitemap generation path and check how alternate media links are represented; done means the sitemap includes the expected xhtml:link element while preserving the existing URL entry.