@vercel/next by default now bundles pages into optimized functions, minimizing bootup time and increasing overall application throughput.
When legacy routes are added in now.json or vercel.json, they cause conflicts with this optimization, so it is opted-out.
Migrate from using legacy routes to the new rewrites, redirects, and headers configurations in your now.json or vercel.json file or leverage them directly in your next.config.js with the built-in custom routes support