await without ConfigureAwait(false) continues on a different thread

I have a WinForms app, and I have some code that needs to run on the UI thread. However, the code after the await runs on a different thread.

protected override async void OnHandleCreated(EventArgs e)
{
    base.OnHandleCreated(e);

    // This runs on the UI thread.
    mainContainer.Controls.Clear();

    var result = await DoSomethingAsync();

    // This also needs to run on the UI thread, but it does not.
    // Instead it throws an exception:
    // "Cross-thread operation not valid: Control 'mainContainer' accessed from a thread other than the thread it was created on"
    mainContainer.Controls.Add(new Control());
}

I also tried explicitly adding ConfigureAwait(true), but it makes no difference. My understanding was that if I omit ConfigureAwait(false), then the continuation should run on the original thread. Is this incorrect in some situations?

I've also noticed that if I add a control to the collection before the await, then the continuation magically runs on the correct thread.

protected override async void OnHandleCreated(EventArgs e)
{
    base.OnHandleCreated(e);

    // This runs on the UI thread.
    mainContainer.Controls.Add(new Control());
    mainContainer.Controls.Clear();

    var result = await DoSomethingAsync();

    // This also runs on the UI thread now. Why?
    mainContainer.Controls.Add(new Control());
}

My question is:

1. Why is this happening?
2. How do I convince the continuation to run on the UI thread (ideally without doing my hack of adding a control and removing it)?

For reference, here are the important parts of DoSomethingAsync. It submits an HTTP request using RestSharp.

protected async Task DoSomethingAsync()
{
    IRestRequest request = CreateRestRequest();

    // Here I await the response from RestSharp.
    // Client is an IRestClient instance.
    // I have tried removing the ConfigureAwait(false) part, but it makes no difference.
    var response = await Client.ExecuteTaskAsync(request).ConfigureAwait(false);

    if (response.ResponseStatus == ResponseStatus.Error)
        throw new Exception(response.ErrorMessage ?? "The request did not complete successfully.");

    if (response.StatusCode >= HttpStatusCode.BadRequest)
        throw new Exception("Server responded with an error: " + response.StatusCode);

    // I also do some processing of the response here; omitted for brevity.
    // There are no more awaits.
}