That’s interesting. I had seen a similar case but it was a Hugo website. Check it out here:
Netlify CLI sets up a proxy to your SSG’s development server, which is why it starts 2 servers - your SSG’s development server and it’s own server. The ‘own’ server takes the request → sends it to the SSG’s server → receives a response from there → sends it back to the original requestor (in most cases the browser). So, I don’t think NextJS renders the content to the out folder when you run its development server which is why I said, that’s not from where CLI is reading the content.