RestSharp ignoring system proxy (for example Fiddler) on .NET Core

I want check the http traffic with Fiddler, but no any http traffic captured, my testing codes:

private static void ByRestSharp()
{
    var restClient = new RestClient("https://jsonplaceholder.typicode.com");
    var request = new RestRequest("posts", Method.GET);
    var response = restClient.Get<List<Post>>(request);
    Console.WriteLine("{0} posts return by RestSharp.", response.Data.Count);
}

But after I changed to use HttpClient, Fiddler can capture the http traffic, sample codes:

private static void ByHttpClient()
{
    var httpClient = new HttpClient();
    using (var req = new HttpRequestMessage(HttpMethod.Get, "https://jsonplaceholder.typicode.com/posts"))
    using (var resp = httpClient.SendAsync(req).Result)
    {
        var json = resp.Content.ReadAsStringAsync().Result;
        var users = SimpleJson.SimpleJson.DeserializeObject<List<Post>>(json);
        Console.WriteLine("{0} posts return by HttpClient.", users.Count);
    }
}

Is this a issue of RestSharp or Fiddler?