Promise.allに終了済みのPromiseを渡しても、エラーなく素通りする

const sleep = msec =>
  new Promise(resolve =>
    setTimeout(() => {
      console.log(`sleep end ${msec}`);
      resolve();
    }, msec)
  );
const exec = async () => {
  console.log(`start ${new Date()}`);
  const promise1 = sleep(1000);
  await sleep(3000);
  console.log("begin promise all");
  await Promise.all([promise1]);
  console.log(`end ${new Date()}`);
};
exec();

結果

start Wed Sep 18 2019 11:28:44 GMT+0900 (Japan Standard Time)
sleep end 1000
sleep end 3000
begin promise all
end Wed Sep 18 2019 11:28:47 GMT+0900 (Japan Standard Time)