lol hold on… actually I solved the mystery.
check your robots.txt! you are blocking static assets from Google Bot here:
Disallow: /_next/
Disallow: /api/
Disallow: /static/
and that means Google cannot render your page without static assets (JS, CSS) and it can only see empty page with no content!
Solution:
remove those lines and enjoy SEO. However I still recommend going with Next.js and SSR! take care!