signIn with provider redirecting to default sign in page instead of provider sign in page

[alessandrojcm]

Hi there, I'm having this issue when trying to redirect directly to a configured provider sign-in page; this used to work correctly in version 4.0.6, but now with version 4.1.2 it works correctly with yarn vercel dev but it doesn't work when deployed to Vercel.

This is how I'm calling signIn

// should go directly to the fusionauth login page
signIn(
      'fusionauth',
      {
          callbackUrl: '/onboarding',
      },
      {
          response_mode: 'form_post',
      }
  )

This is the relevant NextAuth config

export default NextAuth({
// fusionauth provider config as per the docs
providers,
// debug, cookie signing key, etc
callbacks: {
// bunch of callback configurations to handle session refresh not relevant to this matter
// redirect callback to handle relative urls
 redirect({ url, baseUrl }): Awaitable<string> {
            return Promise.resolve(new URL(url, baseUrl).toString());
    },
})

When deployed to Vercel, if I click the button that calls signIn this happens:

The NEXTAUTH_URL is set to the custom domain the deployment was assigned to, like I said this use to work correctly on 4.0.6 and by reading the changelog I can't seem to find any changes that should've altered this behavior.

Any ideas? The Next.js version is 12.0.9, and I have a simple middleware; I can post the code of that middleware if it is relevant.
EDIT: Just downgraded to 4.0.6 and it also happens, maybe there is something off with Vercel?

[alessandrojcm]

Ok so I've resorted to replacing next-auth's default sign-in page with my custom one and adding a permanent redirect to my next.config.js (/login to /auth/login). Clicking signIn('fusionauth') from here works as expected, but I am still curious as to why this broke in the first place.