将LCObject先保存在本地，再save()到云端是空的？

我先把一个LCObject先保存在本地，点击同步按钮后再调用save()，但保存到云端的没有内容。保存前toString看是有内容的。这是为什么？
I/flutter (19403): 日志拦截: sync = {"_type":"Pointer","className":"keijnz","objectId":null,"sportstype":0,"start_time":1637413867,"end_time":1637413870,"duration":3,"max_hr":0,"calories_max":0.0,"pressure_max":0.0}
I/flutter (19403): 日志拦截: *** Request ***
I/flutter (19403): 日志拦截: uri: https://voaji3og.api.lncldglobal.com/1.1/classes/keijnz
I/flutter (19403): 日志拦截: method: POST
I/flutter (19403): 日志拦截: responseType: ResponseType.json
I/flutter (19403): 日志拦截: followRedirects: true
I/flutter (19403): 日志拦截: connectTimeout: 0
I/flutter (19403): 日志拦截: sendTimeout: 0
I/flutter (19403): 日志拦截: receiveTimeout: 0
I/flutter (19403): 日志拦截: receiveDataWhenStatusError: true
I/flutter (19403): 日志拦截: extra: {}
I/flutter (19403): 日志拦截: headers:
I/flutter (19403): 日志拦截: X-LC-Id: vOaJI3OgY1lJ4dxFDQERvB3O-MdYXbMMI
I/flutter (19403): 日志拦截: User-Agent: LeanCloud-Flutter-SDK/0.7.6
I/flutter (19403): 日志拦截: X-LC-Sign: f61578e069c709d4eba6e602d650d04b,1637413885827
I/flutter (19403): 日志拦截: X-LC-Session: z18dnd0wjbg1hhg81adpn6ja2
I/flutter (19403): 日志拦截: content-type: application/json
I/flutter (19403): 日志拦截: data:
I/flutter (19403): 日志拦截: {}
I/flutter (19403): 日志拦截:
I/flutter (19403): 日志拦截: *** Response ***
I/flutter (19403): 日志拦截: uri: https://voaji3og.api.lncldglobal.com/1.1/classes/keijnz
I/flutter (19403): 日志拦截: statusCode: 201
I/flutter (19403): 日志拦截: headers:
I/flutter (19403): 日志拦截: connection: keep-alive
I/flutter (19403): 日志拦截: location: /1.1/classes/keijnz/6198f3fdd5b1eb115347aeb6
I/flutter (19403): 日志拦截: cache-control: no-cache,no-store
I/flutter (19403): 日志拦截: date: Sat, 20 Nov 2021 13:11:25 GMT
I/flutter (19403): 日志拦截: transfer-encoding: chunked
I/flutter (19403): 日志拦截: content-type: application/json;charset=utf-8
I/flutter (19403): 日志拦截: pragma: no-cache
I/flutter (19403): 日志拦截: server: openresty
I/flutter (19403): 日志拦截: Response Text:
I/flutter (19403): 日志拦截: {"objectId":"6198f3fdd5b1eb115347aeb6","createdAt":"2021-11-20T13:11:25.613Z"}
I/flutter (19403): 日志拦截:

直接用你们文档中的例子就可以复现这个问题

void lc_test() async
{
LCObject object = LCObject("Todo");
object['title'] = '马拉松报名';
object['priority'] = 2;
object['owner'] = LCUser.getCurrent();
String serializedString = object.toString();

LCObject newObject = LCObject.parseObject(serializedString);
await newObject.save();
}