The following code works as expected – both services start at the same time.

List<Service> services = [ new Service(), new Service() ];

foreach (Service service in services)
{
    Task task = service.StartAsync();
}

Console.ReadLine();

class Service
{
    public async Task StartAsync()
    {
        await Console.Out.WriteLineAsync($"{DateTime.Now.ToString("HH:mm:ss")}tStarted");

        await Task.Delay(TimeSpan.FromSeconds(1));

        //Simulate some synchronous work.
        Thread.Sleep(10_000);

        await Console.Out.WriteLineAsync($"{DateTime.Now.ToString("HH:mm:ss")}tFinished");
    }
}

Output is:

20:32:25     Started
20:32:25     Started
20:32:35     Finished
20:32:35     Finished

But when you change the delay from TimeSpan.FromSeconds(1) to TimeSpan.FromMicroseconds(1), the output is:

20:40:10     Started
20:40:20     Finished
20:40:20     Started
20:40:30     Finished

Why does changing the delay affect behavior? Is there a race condition going on?