protobuf3 映射不可修改？

Question

protobuf 消息定义：使用 protobuf-3.6.1

syntax = "proto3";

message UserMessage {
    message Int64List {
        repeated int64 value = 1;
    };
    int64 uid = 1;
    int64 gid = 2;
    repeated int32 cat = 3;
    map<string, int32> simple_map = 4;
    map<string, Int64List> map_list = 5;
};

cp代码：

UserMessage message;
auto m = message.mutable_simple_map();
(*m)["aaa"] = 2;
(*m)["bbb"] = 5;
{
    std::cout << message.ShortDebugString() << std::endl;
    auto a = message.simple_map();
    for (auto& b : a) {
        std::cout << "new  " << b.first << ":" << b.second << std::endl;
    }
}
(*m)["aaa"] = 20;
(*m)["bcb"] = 5;
{
    std::cout << message.ShortDebugString() << std::endl;
    auto a = message.simple_map();
    for (auto& b : a) {
        std::cout << "new  " << b.first << ":" << b.second << std::endl;
    }
}

输出：

simple_map { key: "aaa" value: 2 } simple_map { key: "bbb" value: 5 }
new  bbb:5
new  aaa:2
simple_map { key: "aaa" value: 2 } simple_map { key: "bbb" value: 5 }
new  bbb:5
new  aaa:20
new  bcb:5

我用地图结构声明了一个消息，并插入一些对并打印。然后我更新了其中一些并再次打印。结果对我来说很奇怪。为什么消息中的地图没有更新？但是独立打印是正确的吗？