I was able to reproduce this based your vercel.json file
The builds property is deprecated and degrades a few things, including client side navigation it seems. The recommended way to implement this is using Vercel’s Monorepo Support
- Delete your vercel.json file
- Set the framework to Next.js and the root directory to
/frontend - Create a separate project on Vercel for your backend. This can be the same git repository, just with a different root directory set.
