I’ve concluded that NextJS rewrites + ISR for those pages is not supported on Netlify. That’s fine because I have a new approach which does make ISR working (as seen in your deploy log and verified on my site).
This new approach does reveal a new problem, but since it’s a problem that isn’t directly related to the old one, I’ve created a new support forums ticket with a clear explanation and example of the problem here: NextJS rewrite not working with SSR.
Thanks for your help anyway!