jest.spyOn not calling mock implementation

Question

I'm trying to mock a function used by another function which is being tested.

Unfortunately, the original function is called and not the mock.

Attached is a minimal reimplementation of the problem:

sandbox.ts:

export function* myGen(): Generator<string> {
  yield "Hello";
  yield "World";
}

export async function* myAsyncGen() {
  yield* myGen();
}

export async function myFn() {
  const gen = myAsyncGen();

  for await (const chunk of gen) {
    console.log("chunk", chunk);
  }
}

sandbox.test.ts:

import * as sandboxMod from "./sandbox";
const { myFn } = sandboxMod;

describe("sandbox", () => {
  it("should call the mock", async () => {
    let called = false;
    const fakeAsyncGen = async function* () {
      called = true;

      yield "Yo!";
    };

    jest.spyOn(sandboxMod, "myAsyncGen").mockImplementation(fakeAsyncGen);

    await expect(myFn()).resolves.not.toThrow();

    expect(called).toBe(true);
  });
});

Any idea how I can make it work?