"trailingSlash" causing CORS errors

Link to the code that reproduces this issue

https://github.com/tippfelher/nextjs-redirect-issue

To Reproduce

Open https://nextjs-permanent-redirect-issue.vercel.app/something
The response header correctly contains Access-Control-Allow-Origin: *

Open https://nextjs-permanent-redirect-issue.vercel.app/something/
The response header contains no Access-Control-Allow-Origin: *

Current vs. Expected behavior

Open https://nextjs-permanent-redirect-issue.vercel.app/something/
The response header also contains Access-Control-Allow-Origin: *

Configuring https://nextjs.org/docs/pages/api-reference/next-config-js/trailingSlash is obviously no solution.

Provide environment information

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

Not sure

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

Vercel (Deployed), Other (Deployed)

Additional context

Repo for reproduction: https://github.com/tippfelher/nextjs-redirect-issue

1. Added route (/src/app/something/route.ts)
2. Added headers in nextjs.config.js

Start with the reproduction repository and inspect src/app/something/route.ts and nextjs.config.js, then compare the deployed responses for /something and /something/. The fix is complete when both responses include Access-Control-Allow-Origin: *; verify against the linked deployment URLs.