@netlify/plugin-nextjs 5.9.3 breaks client-side navigation

[aleehedl]

When my Next.js site using pages router has i18n enabled, has middleware and has rewrites, the client-side navigation doesn't work properly for non-default locales (fi) with @netlify/plugin-nextjs version 5.9.3 and 5.9.4.

What happens:

1. Open https://deploy-preview-4--jovial-salmiakki-51d4db.netlify.app/fi
2. Hover on link "toinen" and see (in devtools network tab) how 404 is returned in the prefetch
3. Click on link "toinen" -> a full page reload happens

What I expected to happen:

• In step 2 I expect the prefetch to return the page props instead of 404.
• In step 3 I expect the client-side navigation to work.

If you switch to the default locale (English) the same does not happen. Also, when you downgrade to plugin version 5.9.2 the problem disappears: https://deploy-preview-3--jovial-salmiakki-51d4db.netlify.app/fi

Minimal reproduction repository here:
https://github.com/aleehedl/netlify-next-plugin-error

[aleehedl]

This conversation sounds like it is related to the same problem:
https://answers.netlify.com/t/404-on-all-routes-when-navigating-on-client-side-after-updating-netlify-plugin-nextjs/137718