Broken metadata on custom admin routes

Describe the Bug

When customising admin routes, the page title and favicon present as if we've hit a 404, rather than an expected route.

Broken custom route metadata:

Expected default route metadata:

Link to the code that reproduces this issue

https://github.com/damnsamn/blank-payload-project/tree/issue/custom-route-meta

Reproduction Steps

1. Customise admin.routes.login
2. Add a custom favicon via admin.meta.icons
3. Navigate to payload's login screen, and see the default favicon and "Not Found" page title.
4. Log in to see the custom favicon and correct page title

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

area: core

Environment Info

Binaries:
  Node: 22.12.0
  npm: 10.9.0
  Yarn: N/A
  pnpm: 9.15.6
Relevant Packages:
  payload: 3.27.0
  next: 15.2.0
  @payloadcms/db-mongodb: 3.27.0
  @payloadcms/email-nodemailer: 3.27.0
  @payloadcms/graphql: 3.27.0
  @payloadcms/next/utilities: 3.27.0
  @payloadcms/payload-cloud: 3.27.0
  @payloadcms/richtext-lexical: 3.27.0
  @payloadcms/translations: 3.27.0
  @payloadcms/ui/shared: 3.27.0
  react: 19.0.0
  react-dom: 19.0.0
Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 24.0.0: Mon Aug 12 20:52:31 PDT 2024; root:xnu-11215.1.10~2/RELEASE_ARM64_T6030
  Available memory (MB): 18432
  Available CPU cores: 12