Next Auth ignoring Next Auth env on proxy domains

[...nextauth].ts

export default async function auth(req: NextApiRequest, res: NextApiResponse) {
  if (process.env.VERCEL) {
    // prefer NEXTAUTH_URL, fallback to x-forwarded-host
    req.headers['x-forwarded-host'] =
      process.env.NEXTAUTH_URL ?? req.headers['x-forwarded-host']
  }

  return await NextAuth(req, res, authOptions())
}