move_base/global_costmap/costmap subscriber not receiving data (but RVIZ and rostopic do)

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Hello,

I am experiencing a problem with ROS Melodic (Ubuntu 18.04, x86):

I am have installed the Tiago packages and I am moving the robot in a simulated environment using the move_base actions. I am trying to obtain the OccupancyGrid message published by the topic move_base/global_costmap/costmap but my subscriber does not seem to be working (I have coded a ROS_INFO call inside the callback and the message never shows up).

In particular, I have written a map_saver class, which has a method save_map() I use to subscribe to the topic and save the OccupancyGrid inside a member variable. I need the map to be saved before I can go on with my computations, but in practice the map is never saved (I never exit the while loop in the main reported below).

Here is the code of the main():

int main(int argc, char** argv) {
    ros::init(argc, argv, "map_elab");
    ros::NodeHandle n;
    ros::Rate r(10);

    map_saver ms(n);
    ROS_INFO("map_saver object instantiated");
    ms.save_map();
    while (!ms.map_is_available() && ros::ok()) { //NEVER EXITING THIS LOOP
        r.sleep();
        ros::spinOnce;
    }
    //---once the map is saved, do something with it---

Here is the skeleton of the map_saver class:

class map_saver {
    private:
    ros::NodeHandle _n;
    nav_msgs::OccupancyGrid _global_map;
    ros::Subscriber _map_subscriber;
    bool _map_is_saved = false;

    public:
    map_saver(ros::NodeHandle n)
        :_n(n) {}
    //...
    void save_map()
    {
        /* STEP 1: move to a waypoint at the end of the corridor using move_base*/
        //...
        /* STEP 2: subscribe to topic /move_base/global_map and save the global map */
        _map_subscriber = _n.subscribe("move_base/global_costmap/costmap", 5, &map_saver::save_map_callback, this);
    }
    void save_map_callback(const nav_msgs::OccupancyGrid::ConstPtr& msg) 
    {
        //we need to make sure the callback is called exactly once!
        ROS_INFO("save_map callback is saving the map...");
        _global_map = *msg;
        _map_subscriber.shutdown();
        _map_is_saved = true;
        ROS_INFO("save_map callback deactivated. Map is saved.");
    }
    const nav_msgs::OccupancyGrid& get_map()
    {return _global_map;}

    bool map_is_available()
    {return _map_is_saved;};

I have no clue as why this is happening, since both RViz and rostopic echo return a proper OccupancyGrid object. What am I getting wrong?