Generator with double yield problem

I have questions
1

Basically why this does not work as expected?

function* gen() {
  while (true) {
    const input = yield;

    yield 5 + input;
  }
}

const values = [];
const geng = gen();
for (let index = 0; index < 5; index++) {
  geng.next(index);
  values.push(geng.next());
}
console.log(values);
2

I've updated MDN: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/yield , see big red box about asymmetry.