EF Core 2.1 In memory DB not updating records

I'm using the in memory database provider for integration tests however I don't seem to be able to update a record. I've run the same code against a real SQL database and everything gets updated fine. Here is my test fixture code.

Test Fixture:

public class TestFixture<TStartup> : IDisposable
{
    private readonly TestServer _testServer;
    public HttpClient TestClient { get; }
    public IDatabaseService DbContext { get { return _testServer.Host.Services.GetService<DatabaseService>(); } }

    public TestFixture() : this(Path.Combine("src")) { }

protected TestFixture(string relativeTargetProjectPatentDir)
{
    Environment.SetEnvironmentVariable("ASPNETCORE_ENVIRONMENT", "Testing");

    var builder = new WebHostBuilder()
        .ConfigureServices(services =>
        {
            services.AddDbContext<DatabaseService>(options =>
                options.UseInMemoryDatabase("TestDB")
                .EnableSensitiveDataLogging());
        })
        .UseEnvironment("Testing")
        .UseStartup<Startup>();

    _testServer = new TestServer(builder)
    {
        BaseAddress = new Uri("http://localhost:5010")
    };

    TestClient = _testServer.CreateClient();
    TestClient.BaseAddress = _testServer.BaseAddress;
}

    public void Dispose()
    {
        TestClient.Dispose();
        _testServer.Dispose();
    }
}

I've spent most of the day googling this and not come across any other people talking about it so I'm assuming its probably my issue rather than a EF bug. I'm sure someone would have noticed a DB that you can't update.

It turned out that changing the lifetime of my DbContext in my test fixture to singleton solved my issue.