The set_result() of Future doesn't end the await after the loop of create_task()

Question

I write a Python program involving a loop of create_task() that calls async functions. In each task, it waits for another thread to set the result for a Future object.

async def run(self) -> int:
    cur_ts = 0
    tasks: List[asyncio.Task] = []
    while not self.requests.empty():
        req = self.requests.get()
        self.num_reqs += 1
        t = asyncio.create_task(self._send_request(req))
        tasks.append(t)
        interval = req.get_timestamp() - cur_ts
        cur_ts = req.get_timestamp()
        # Sleep for x ms
        await asyncio.sleep(interval / 1000)
    await asyncio.gather(*tasks)
    accuracy = self.num_corrects.get() / self.num_reqs
    failure = self.num_failures.get() / self.num_reqs
    print("Accuracy is ", accuracy, ", failure rate is ", failure)
    return 0

async def _send_request(self, req: Request) -> None:
    err = self.sched.receive(req)
    if err == 0:
        res = await req.get_result()
        if res == 1:
            self.num_corrects.increment()
    else:
        self.num_failures.increment()

I encountered a strange problem that before the end of the loop in run(), when the result is set, _send_request() can proceed after the await. But once the loop ends, _send_request() will be stuck even if the result is set.

The set_result() and get_result() in Request are:

def set_result(self, result: int):
    if not self.result.done():
        self.result.set_result(result)

async def get_result(self) -> int:
    return await self.result

The Future object is created in receive(). I have checked the id of the event loop and I found all the created tasks are in the same loop.

Hence, I wonder what can be the cause of this issue?

Thanks!