Task.Yield() no longer reliable for testing?

private async Task RunAsync(CancellationToken stopToken)
{
    while (!stopToken.IsCancellationRequested)
    {
        await entryChannelReader.WaitToReadAsync();

        while (entryChannelReader.TryRead(out var entry)
            payload.AppendEntry(entry);

        if (/*Delivery not yet needed*/)
            continue;

        if (/*Too soon since last delivery*/)
            await _systemClock.WaitAsync(/*Remaining time until next delivery*/, stopToken);

        await deliveryManager.DeliveryPayloadAsync(payload);

        payload.Clear();
    }
}