Bug with metadata alternates when using URL

Link to the code that reproduces this issue

https://github.com/votemike/alternate-urls

To Reproduce

• Start the application in development
• Visit the homepage
• Inspect the alternate URLs in the HTML
• Notice the US one does not contain the US suffix

Current vs. Expected behavior

Expected:
I expect the HTML to contain <link rel="alternate" hreflang="en-GB" href="https://example.com"> and <link rel="alternate" hreflang="en-US" href="https://example.com/us">
Current:
The HTML does not contain the US suffix on the URL

Provide environment information

Operating System:
  Platform: darwin
  Arch: x64
  Version: Darwin Kernel Version 21.6.0: Wed Apr 24 06:02:02 PDT 2024; root:xnu-8020.240.18.708.4~1/RELEASE_X86_64
  Available memory (MB): 16384
  Available CPU cores: 4
Binaries:
  Node: 18.18.0
  npm: 9.8.1
  Yarn: 1.22.10
  pnpm: N/A
Relevant Packages:
  next: 15.0.0-canary.94 // Latest available version is detected (15.0.0-canary.94).
  eslint-config-next: N/A
  react: 19.0.0-rc-3208e73e-20240730
  react-dom: 19.0.0-rc-3208e73e-20240730
  typescript: 5.3.3
Next.js Config:
  output: N/A

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

Metadata

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

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

Additional context

I found this bug in Next 14.1.1
Casting the URL to a string gives me the output I expect. Not sure why the HTML doesn't reflect this.

Start with the reproduction at https://github.com/votemike/alternate-urls, run it in the reported Next.js environments, and inspect the rendered alternate links on the homepage. Compare the URL value with its string-cast output; done means the HTML includes the expected en-GB URL and the en-US URL with the /us suffix.