Why is my async/await with CancellationTokenSource leaking memory?

I have a .NET (C#) application that makes extensive use of async/await. I feel like I've got my head around async/await, but I'm trying to use a library (RestSharp) that has an older (or perhaps I should just say different) programming model that uses callbacks for asynchronous operations.

RestSharp's RestClient class has an ExecuteAsync method that takes a callback parameter, and I wanted to be able to put a wrapper around that which would allow me to await the whole operation. The ExecuteAsync method looks something like this:

public RestRequestAsyncHandle ExecuteAsync(IRestRequest request, Action<IRestResponse> callback);

I thought I had it all working nicely. I used TaskCompletionSource to wrap the ExecuteAsync call in something that I could await, as follows:

public static async Task<T> ExecuteRequestAsync<T>(RestRequest request, CancellationToken cancellationToken) where T : new()
{
    var response = await ExecuteTaskAsync(request, cancellationToken);

    cancellationToken.ThrowIfCancellationRequested();

    return Newtonsoft.Json.JsonConvert.DeserializeObject<T>(response.Content);
}

private static async Task<IRestResponse> ExecuteTaskAsync(RestRequest request, CancellationToken cancellationToken)
{
    var taskCompletionSource = new TaskCompletionSource<IRestResponse>();

    var asyncHandle = _restClient.ExecuteAsync(request, r => 
    {
        taskCompletionSource.SetResult(r); 
    });

    cancellationToken.Register(() => asyncHandle.Abort());

    return await taskCompletionSource.Task;
}

This has been working fine for most of my application.

However, I have one part of the application that does hundreds of calls to my ExecuteRequestAsync as part of a single operation, and that operation shows a progress dialog with a cancel button. You'll see th