Keystatic Admin Page - 404 on vercel production deployment

Thankyou @simonswiss and team, Keystatic is a boon for static websites.
I was using Keystatic happily in local mode, however, I needed to access my website remotely, so I switched to GitHub mode.
I successfully ran and used GitHub mode in dev environment, but when deployed, the Keystatic login page appears, but on login, the page goes 404.
My deployment service is Vercel, and I am using Vercel adapter in my Astro.js project.

before login 👆

after login 👆

Please help me fix this, I searched and tried all that was possible, but nothing worked.

I have ensured:

1. .env keys in Vercel
2. correct callback url in GitHub app
3. correct config in Keystatic and Astro.js configs

Btw I often get a blank white page on http://localhost:4321/keystatic but it fixes after re runs or after sometime

Start by reproducing the production failure at the /keystatic entry point with the Astro.js Vercel adapter and GitHub mode enabled. Compare the deployed login and callback behavior with the working development setup, including the GitHub callback URL and Vercel environment keys. Done means a production GitHub login reaches the Keystatic admin page instead of returning 404.