aspnetcore: HttpClient.PostJsonAsync deserialize json failed

Describe the bug

When I use below code in razor page to post a http request. The deserialized object is not correct. There is no exception happened.

billInfo = await Http.PostJsonAsync<BillInfo>("api/TMobileBill", filePath);

I then changed to use Http.PostAsync to get the response string, then deserialize it by using Newtonsoft library. It works fine.

        var myContent = Newtonsoft.Json.JsonConvert.SerializeObject(filePath);
        var buffer = System.Text.Encoding.UTF8.GetBytes(myContent);
        var byteContent = new ByteArrayContent(buffer);
        byteContent.Headers.ContentType = new System.Net.Http.Headers.MediaTypeWithQualityHeaderValue("application/json");
        var response = await Http.PostAsync("api/TMobileBill", byteContent);
        var responseStr = await response.Content.ReadAsStringAsync();
        billInfo = Newtonsoft.Json.JsonConvert.DeserializeObject<BillInfo>(responseStr);
        StateHasChanged();

The response json string in my problem is:

{"period":"May 09, 2019 - Jun 08, 2019","dueDate":"Jul 01, 2019","totalAmount":326.03,"billInfoPerPerson":[{"name":"A B","phonesInfo":[{"phoneNumber":"(425) 111-1111","planHost":false,"usage":4.84,"regularCharge":30.0,"otherCharge":0.0,"totalCharge":0.0},{"phoneNumber":"(425) 222-2222","planHost":false,"usage":5.48,"regularCharge":30.0,"otherCharge":0.0,"totalCharge":0.0}],"totalAmount":60.0},{"name":"C D","phonesInfo":[{"phoneNumber":"(425) 333-3333","planHost":false,"usage":1.08,"regularCharge":20.0,"otherCharge":0.0,"totalCharge":0.0},{"phoneNumber":"(425) 444-4444","planHost":false,"usage":1.47,"regularCharge":20.0,"otherCharge":0.0,"totalCharge":0.0}],"totalAmount":40.0},{"name":"E F","phonesInfo":[{"phoneNumber":"(425) 555-5555","planHost":false,"usage":1.05,"regularCharge":20.0,"otherCharge":28.03,"totalCharge":0.0},{"phoneNumber":"(425) 666-6666","planHost":false,"usage":2.15,"regularCharge":30.0,"otherCharge":0.0,"totalCharge":0.0}],"totalAmount":78.03},{"name":"G H","phonesInfo":[{"phoneNumber":"(425) 777-7777","planHost":false,"usage":1.52,"regularCharge":20.0,"otherCharge":17.5,"totalCharge":0.0},{"phoneNumber":"(425) 888-8888","planHost":true,"usage":5.27,"regularCharge":30.0,"otherCharge":20.5,"totalCharge":0.0}],"totalAmount":88.0},{"name":"I J","phonesInfo":[{"phoneNumber":"(425) 999-9999","planHost":false,"usage":3.67,"regularCharge":30.0,"otherCharge":0.0,"totalCharge":0.0},{"phoneNumber":"(425) 000-0000","planHost":false,"usage":3.05,"regularCharge":30.0,"otherCharge":0.0,"totalCharge":0.0}],"totalAmount":60.0}],"warning":"There are $3 account service fee. Make sure it is expected. "}

The corresponding class is:

    public class PersonBillInfo
    {
        public string Name;
        public List<PhoneBillInfo> PhonesInfo;
        public float TotalAmount;
    }

    public class PhoneBillInfo
    {
        public string PhoneNumber;
        public bool PlanHost;
        public float Usage;
        public float RegularCharge;
        public float OtherCharge;
        public float TotalCharge;
    }

    public class BillInfo
    {
        public string Period;
        public string DueDate;
        public float TotalAmount;
        public IEnumerable<PersonBillInfo> BillInfoPerPerson;
        public string Warning;
    }

To Reproduce

Steps to reproduce the behavior:

1. Using this version of ASP.NET Core ‘SDK 3.0.100-preview6-012264’
2. Implement a controller action to respond the json string as above.
3. From razor page, use below code to call the controller action.

var billInfo = Http.PostJsonAsync<BillInfo>("api/TMobileBill", filePath);

1. Evaluate the billInfo object. All the members of billInfo object are null.

Expected behavior

The members of billInfo object have values.