url fragment is missed in post response

// react frontend
    const response = await fetch(
      "http://localhost:4242/create-checkout-session#abc",
      {
        method: "POST",
      }
    );

console.log('response.url', response.url); // 'https://checkout.stripe.com/xyz'

// create a stripe checkout session in express server
  res.redirect(303, session.url); // 'https://checkout.stripe.com/xyz#abc...'

I don’t understand why url fragment is missed from response of fetch post request, can anyone explain this please?