message_filters in C++ not subscribing to topics when implemented in a class

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Hi all, I am having some issues with message filters, but I am not sure if I am doing something wrong or if there is a bug somewhere.

After some difficulties, I decided to implement a couple of minimal examples to describe the problem.

So, the toy example is very simple, I have a publisher node that publishes on two topics (topic1 and topic2) a custom-defined IntStamped message. They have exactly the same timestamp, so there are no issues in the generated messages.

Then, I have implemented two C++ nodes using message_filters, the first one wraps everything in a class, while the second doesn't.

Here are the nodes.

Whit the classes:

#include "message_filters/subscriber.h"
#include "message_filters/time_synchronizer.h"
#include "boundingbox_msg/msg/int_stamped.hpp"

using namespace  message_filters;

using std::placeholders::_1;
using std::placeholders::_2;

class MinimalSyncClass : public rclcpp::Node {
public:
    MinimalSyncClass() : Node("minimal_sync_class") {
        Subscriber<boundingbox_msg::msg::IntStamped> sub1(this, "topic1");
        Subscriber<boundingbox_msg::msg::IntStamped> sub2(this, "topic2");
        TimeSynchronizer<boundingbox_msg::msg::IntStamped, boundingbox_msg::msg::IntStamped> sync(
                sub1,
                sub2,
                10);
        sync.registerCallback(std::bind(&MinimalSyncClass::callback, this,
                                        std::placeholders::_1,
                                        std::placeholders::_2));
        RCLCPP_INFO_STREAM(this->get_logger(), "Init complete");
    }
private:
    void callback(
            const boundingbox_msg::msg::IntStamped::ConstSharedPtr& msg1,
            const boundingbox_msg::msg::IntStamped::ConstSharedPtr& msg2) const {
        RCLCPP_INFO_STREAM(this->get_logger(), "Result: "<<msg1->value + msg2->value );
    }
};


int main(int argc, char *argv[]) {
    rclcpp::init(argc, argv);
    rclcpp::spin(std::make_shared<MinimalSyncClass>());
    rclcpp::shutdown();
    return 0;
}

And this one without the class:

#include "message_filters/subscriber.h"
#include "message_filters/time_synchronizer.h"
#include "boundingbox_msg/msg/int_stamped.hpp"

using namespace  message_filters;

using std::placeholders::_1;
using std::placeholders::_2;

void callback(
        const boundingbox_msg::msg::IntStamped::ConstSharedPtr& msg1,
        const boundingbox_msg::msg::IntStamped::ConstSharedPtr& msg2) {
    RCLCPP_INFO_STREAM(rclcpp::get_logger("minimal_sync"), "Result: "<<msg1->value + msg2->value );
}

int main(int argc, char** argv)
{
    rclcpp::init(argc, argv);
    auto nh = std::make_shared<rclcpp::Node>("minimal_sync");
    Subscriber<boundingbox_msg::msg::IntStamped> sub1(nh.get(), "topic1");
    Subscriber<boundingbox_msg::msg::IntStamped> sub2(nh.get(), "topic2");
    TimeSynchronizer<
            boundingbox_msg::msg::IntStamped,
            boundingbox_msg::msg::IntStamped> sync(sub1, sub2, 10);
    sync.registerCallback(std::bind(&callback, _1, _2));
    RCLCPP_INFO(rclcpp::get_logger("minimal_sync"), "Init" );
    rclcpp::spin(nh);
    return 0;
}

If I run all the three nodes together, this is the result in rqt_graph:

As you can see, the one without the class is connected to the topics and it is working properly, while the other with the class it's completely disconnected.

I am honestly lost, and can't figure out where the problem is.