Bug description

When setting up a vercel reverse proxy, the rewrites do not work when deployed to vercel.com.
I created a test repo: https://github.com/MarconLP/posthog-test
And deployed it to: https://posthog-test-marcus.vercel.app/
It works when running nextjs locally with vercel dev

How to reproduce

1. Follow the vercel reverse proxy guide
2. Run vercel dev and see it working locally
3. Deploy to vercel.com
4. It does not work

Additional context

related ticket: https://posthog.com/questions/proxy-return-405

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