When calling auth.api.getSession, why is the user key inside the session object sometimes undefined?

import { auth } from "./auth";
import { headers } from "next/headers";

export default async function Page() {
  const session = await auth.api.getSession({ headers: await headers() });

  if (!session) {
    redirect("/auth/sign-in");
  }

  console.log(session) // user is sometimes undefined inside session, why?
 
  // ...rest of component
}