How to test Automatonymous state machines?

I'm trying to write tests for an Automatonymous state machine, but I'm having a fair bit of trouble getting it right, and I've found very little documentation.

Here's what I have at the moment for one test:

[TestFixture]
public class MyProcessStateMachineTests
{
    InMemoryTestHarness _Harness;
    MyProcessStateMachine _Machine;
    StateMachineSagaTestHarness<MyProcess, MyProcessStateMachine> _Saga;

    [OneTimeSetUp]
    public void ConfigureMessages()
    {
        MessageCorrelation.UseCorrelationId<RequestMyDetails>(x => x.CorrelationId);
        MessageCorrelation.UseCorrelationId<FileAttached>(x => x.CorrelationId);
        MessageCorrelation.UseCorrelationId<PDFGenerated>(x => x.CorrelationId);
        MessageCorrelation.UseCorrelationId<CustomerAttachFile>(x => x.CorrelationId);
        MessageCorrelation.UseCorrelationId<AddCustomerNote>(x => x.CorrelationId);
        MessageCorrelation.UseCorrelationId<EmailPublished>(x => x.CorrelationId);
    }


    [SetUp]
    public void InitializeTestHarness()
    {
        _Harness = new InMemoryTestHarness();
        _Machine = new MyProcessStateMachine( /* snip */ );
        _Saga = _Harness.StateMachineSaga<MyProcess, MyProcessStateMachine>(_Machine);

        _Harness.Start().Wait();
    }

    [TearDown]
    public void StopTestHarness()
    {
        _Harness.Stop();
    }


    [Test]
    public async Task ShouldAttachToCustomer()
    {
        var sagaId = Guid.NewGuid();
        var custId = Guid.NewGuid();
        var fileAttached = BuildFileAttachedMessage(sagaId);

        await _Harness.InputQueueSendEndpoint.Send(BuildStartMessage(sagaId));
        await _Harness.InputQueueSendEndpoint.Send(BuildDetailsReceivedMessage(sagaId));
        await _Harness.InputQueueSendEndpoint.Send(BuildPdfGeneratedMessage(sagaId));
        await _Harness.InputQueueSendEndpoint.Send(fileAttached);

        // Next line is based on [the answer here][1]
        // Once the above messages are all consumed and processed,
        // the state machine should be in AwaitingEmail state
        await _Saga.Match(x =>
            x.CorrelationId == sagaId
                && x.CurrentState == _Machine.AwaitingEmail.Name,
            new TimeSpan(0, 0, 30));

        // Grab the instance and Assert stuff...
    }

    // Snip...
}

Given that the _Saga.Match call finds a match, I would expect that all messages have been processed and I should be able to grab my state machine instance and published events and check their values - but that isn't the case. When I run the tests in the fixture, sometimes the instance I get has consumed and published the expected messages; sometimes it's not quite there yet.

I've tried grabbing my instance using:

var inst = _Saga.Sagas.FirstOrDefault(x => x.Saga.CorrelationId == sagaId);

or grabbing published events with:

var test = _Harness.Published
    .FirstOrDefault(x => x.MessageType == typeof(IAttachFile) && x.Context.CorrelationId == sagaId);

but it doesn't matter that the call to Match succeeded, the state machine instance (and published events) aren't always present.

I'm assuming that the async proccesses from Automatonymous, MassTransit, or test harness is causing the inconsistency. Any help?

Testing with MassTransit, MassTransit.Automatonymous and MassTransit.TestFramework 5.1.2.1528, Automatonymous 4.1.1.102,