Astroのサーバーエンドポイントにおける redirect() メソッドでリダイレクトできない

import { defineMiddleware } from 'astro:middleware'
import { createClient } from '@supabase/supabase-js'

const supabase = createClient(
  import.meta.env.SUPABASE_URL,
  import.meta.env.SUPABASE_ANON_KEY
)

export const onRequest = defineMiddleware(async ({ url, cookies, redirect }, next) => {
  // Cookieからアクセストークンとセッショントークンを取得
  // (保存する処理はのちほど)
  const accessToken = cookies.get('sb-access-token');
  const refreshToken = cookies.get('sb-refresh-token');

  if (accessToken && refreshToken) {
    const { data, error } = await supabase.auth.setSession({
      refresh_token: refreshToken.value,
      access_token: accessToken.value,
    });

    if (['/signup'].includes(url.pathname)) {
      return redirect('/')
    }
  }

  if (!accessToken || !refreshToken) {
    if (url.pathname === '/') {
      return redirect('/signup')
    }
  }

  return next()
})

<form id="form">
  <div>email:</div>
  <div>
    <input type="email" name="email" value="">
  </div>
  <div>password:</div>
  <div>
    <input type="password" name="password" value="">
  </div>
  <button type="submit">送信する</button>
</form>

<script>
  const form = document.getElementById('form');

  if (!form) {
    throw new Error('form not found');
  }
  
  form.addEventListener('submit', async (e) => {
    e.preventDefault()

    const formData = new FormData(form as HTMLFormElement)

    fetch(`/api/auth/signup`, {
      method: 'POST',
      body: formData,
    })
    .then((res) => {
      console.log(res)
    })
  });
</script>

import type { APIRoute } from 'astro'
import { createClient } from '@supabase/supabase-js'

const supabase = createClient(
  import.meta.env.SUPABASE_URL,
  import.meta.env.SUPABASE_ANON_KEY
)

export const POST: APIRoute = async ({ request, cookies, redirect }) => {
  const formData = await request.formData()
  const email = formData.get('email')
  const password = formData.get('password')

  const { data, error } = await supabase.auth.signUp({
    email: email as string,
    password: password as string
  })

  if (data?.session)  {
    const { access_token, refresh_token } = data.session;

    // アクセストークンとリフレッシュトークンをCookieに保存
    cookies.set('sb-access-token', access_token, {
      path: '/',
      httpOnly: true,
      secure: true,
      sameSite: 'strict',
    });
    cookies.set('sb-refresh-token', refresh_token, {
      path: '/',
      httpOnly: true,
      secure: true,
      sameSite: 'strict',
    });
  }

  return redirect('/')
}

fetch(`/api/auth/signup`, {
  method: 'POST',
  body: formData,
})
.then((res) => {
  if (res.redirected) {
    window.location.href = res.url;
  }
})
.catch((error) => {
  console.error(error);
})