Firebase Flatter Firestore:使用startAtDocument的结果不一致

我正在使用Web上的FlatterFireStore进行分页查询。当我尝试使用“startAtDocument”获取结果的第二页时，结果有时会显示一个文档

下面是我的简单测试用例：

Future<void> loadSendQueueSIMPLETTEST(SendQueryType theType) async {
    final num PAGE_LENGTH = 3;
    ObservableList<DocumentSnapshot> sendQueueDocsLocal;

    QuerySnapshot snap;

    // We're doing an initial page query
    snap = await Firestore.instance.collection('departments')
        .document(departmentID)
        .collection('send_queue')
        .orderBy('created')
        .limit(PAGE_LENGTH)
        .getDocuments();

    sendQueueDocsLocal = new ObservableList.of(snap.documents);

    sendQueueDocsLocal.forEach((element) {
      print("GOT SEND QUEUE DOCS: ${element.documentID}");
    });

    DocumentSnapshot startDoc = sendQueueDocsLocal[2];
    print("DOING PREV QUERY startAt: ${startDoc.documentID}");
    snap = await Firestore.instance.collection('departments')
        .document(departmentID)
        .collection('send_queue')
        .orderBy('created')
        .startAtDocument(startDoc)
        .limit(PAGE_LENGTH)
        .getDocuments();

    sendQueueDocsLocal.clear();
    sendQueueDocsLocal.addAll(snap.documents);

    sendQueueDocsLocal.forEach((element) {
      print("SECOND SEND QUEUE DOCS: ${element.documentID}");
    });
    

  }

如果我将开始文档更改为另一个文档（不同的索引），如：

DocumentSnapshot startDoc=sendQueueDocsLocal[1]
结果看起来不错：

GOT SEND QUEUE DOCS: ahkiflypgVFeUgk6e3Kk
GOT SEND QUEUE DOCS: 7NtDiDR5hwSulB8SOou5
GOT SEND QUEUE DOCS: ioIXHJJKsuPXA7g5Bks2

DOING PREV QUERY startAt: 7NtDiDR5hwSulB8SOou5

SECOND SEND QUEUE DOCS: 7NtDiDR5hwSulB8SOou5   <--- This looks right!
SECOND SEND QUEUE DOCS: ioIXHJJKsuPXA7g5Bks2
SECOND SEND QUEUE DOCS: Th3pHayGzqA1y1i3UTSf

获取发送队列文档：ahkiflypgfeugk6e3kk
已获取发送队列文档：7NtDiDR5hwSulB8SOou5
已获取发送队列文档：ioixhjksupxa7g5bks2
正在执行上一个查询开始：7NtDiDR5hwSulB8SOou5
第二个发送队列文档：7NtDiDR5hwSulB8SOou5
GOT SEND QUEUE DOCS: ahkiflypgVFeUgk6e3Kk
GOT SEND QUEUE DOCS: 7NtDiDR5hwSulB8SOou5
GOT SEND QUEUE DOCS: ioIXHJJKsuPXA7g5Bks2

DOING PREV QUERY startAt: 7NtDiDR5hwSulB8SOou5

SECOND SEND QUEUE DOCS: 7NtDiDR5hwSulB8SOou5   <--- This looks right!
SECOND SEND QUEUE DOCS: ioIXHJJKsuPXA7g5Bks2
SECOND SEND QUEUE DOCS: Th3pHayGzqA1y1i3UTSf