Async generator yielding undefined values

[jedel1043]

Taken from https://github.com/tc39/test262/blob/9704d7f22f6342d6c4753ab9a8d62d6725de8c4e/test/language/expressions/await/for-await-of-interleaved.js

To Reproduce

const actual = [];
const iterations = 3;

async function* naturalNumbers(start) {
    let current = start;
    while (current > 0) {
        yield Promise.resolve(current--);
    }
}

async function trigger() {
    for await (const num of naturalNumbers(iterations)) {
        actual.push('A' + num);
    }
}

async function checkAssertions() {
    console.log(actual) // should print P6,P5,A3,P4,P3,A2,P2,P1,A1,P0
}

function countdown(counter) {
    actual.push('P' + counter);
    if (counter > 0) {
        return Promise.resolve(counter - 1).then(countdown);
    } else {
        checkAssertions();
    }
}

trigger();
countdown(iterations * 2);

Expected behavior
Print P6,P5,A3,P4,P3,A2,P2,P1,A1,P0

Current behavior
Prints P6,P5,A3,P4,P3,Aundefined,P2,P1,A2,P0