Issues with Authentication on Sandbox

Hi, I'm trying to set up a demonstration project using the sandbox environment but I'm having troubles authenticating with the API. I initially used the .NET SDK to access the sandbox but when I hit troubles I decided to try and distill the problem down. What I'm observing is that my first attempt to connect to the sandbox API always yields a 401, even when I am explicitly supplying the API Key in the Authorization header. As long as I use the same HttpClient instance, thereafter I can then get through without problems. I've replicated this behavior in Postman, so I don't believe it is .NET specific. Here's the code:

        private static HttpClient client = CreateClient();

        static async Task Main(string[] args)
        {
            await Go(client);
            await Go(client);
            await Go(client);
        }

        private static HttpClient CreateClient()
        {
            var handler = new HttpClientHandler()
            {
                PreAuthenticate = true,
            };

            var httpClient = new HttpClient(handler)
            {
            };
            
            httpClient.DefaultRequestHeaders.Add("Authorization", "Basic {api key goes here}");

            httpClient.BaseAddress = new Uri("https://sandbox.esignlive.com/api/");
            return httpClient;
        }

        private static async Task Go(HttpClient httpClient)
        {
            try
            {
                var package = await httpClient.GetAsync("packages");
                Console.WriteLine($"Got packages: {await package.Content.ReadAsStringAsync()}");
            }
            catch (Exception e)
            {
                Console.WriteLine(e.ToString());
            }
        }

Outcome:

Got packages: {"messageKey":"error.unauthorised.noSession","message":"Failed to retrieve Session","code":401,"name":"Unauthorized"}
Got packages: {"count":0,"results":[]}
Got packages: {"results":[],"count":0}

I am behind a proxy that I think might be part of the issue, but I am not sure. I had to configure the default .NET proxy as such:

Any ideas on what might be gumming up the works? Thanks.