Gulp 'series' and 'parallel' does not signal async completion even though everything works fine

Question

According to their documentation on series() and parallel() and their example, you only need to return a function call. However it doesn't work for me. This is my code:

import gulp from "gulp";

export const test = () => {
    return gulp.parallel(
        async () => console.log("Async done"),
        cb => { console.log("cb done"); cb(); },
    )(); // Without function call () nothing is run
};

// package.json gulp version:
{
  "devDependencies": {
    "gulp": "^4.0.2",
    // ...
  }
  // ...
}

The above code works fine, I can see the tasks being completed in the output. However, there's always a warning at the end:

The following tasks did not complete: test
Did you forget to signal async completion?

I have tried:

• Do exactly like the example and return gulp.parallel() call without calling it. However in that case, nothing is executed and this answer helped: https://stackoverflow.com/a/39902796/653457

• Remove either the async method or the cb method (so only a single task is executed). The warning still exists.

Is this a gulp's bug or did I do something wrong? I am new to Node and gulp.

I am not sure if this is related to the issue but I cannot use this like most documentation and examples:

import gulp, { series, parallel } from "gulp";

Named export 'parallel' not found. The requested module 'gulp' is a CommonJS module, which may not support all module.exports as named exports.

---

Update: for some reason, adding async to the function helps, I assume because Gulp now works on the Promise returned instead (you don't even need to use await on the parallel result, and it's not a Promise anyway):

export const test = async () => { // add async

Update 2:

Thanks to Mark comment, this also works but I have to wrap everything in another series, and technically test is done BEFORE the final task completes:

export const test = (cb) => {
    return gulp.series(
        gulp.parallel(
            async () => { await new Promise(r => setTimeout(r, 1000)); console.log("Async done") },
            cb => { console.log("cb done"); cb(); },
        ),
        async () => cb(), // This is done after test finish
    )();
}

// Output:
// ...
// [12:24:44] Finished 'test' after 1.01 s
// [12:24:44] Finished '<anonymous>' after 1.33 ms