Can't authenticate via NTLM from Linux Kestrel hosted ASP.NET API to IIS hosted ASP.NET API

public HttpResponseMessage CMImport(List<CMImport> Imports)
{
    var client = NTLMHttpClient();
    var result = client.PostAsJsonAsync(_config["CMImport:importurl"], Imports).Result;
    if (result.StatusCode == HttpStatusCode.Unauthorized)
    {
        foreach (var header in result.Headers)
        {
            Console.WriteLine($"{header.Key}: {string.Join(", ", header.Value)}");
        }
    }
    else
    {
        Console.WriteLine("Request succeeded.");
    }

    return result;
}

private HttpClient NTLMHttpClient()
{
    var credentials = new NetworkCredential(_config["CMImport:username"], _config["CMImport:password"]);
    var handler = new HttpClientHandler
    {
        Credentials = credentials,
        ServerCertificateCustomValidationCallback = (message, cert, chain, errors) => { return true; }
    };
    var client = new HttpClient(handler);
    return client;
}