I can't communicate from C++ to GnuRadio with ZeroMQ REQ

I have a C++ code to send commands and change parameters in a GnuRadio flowgraph. To do a simple test I'm trying to send a string from a C++ code and visualize it in GnuRadio.This is my C++ code:

#include <zmq.hpp>#include <iostream>#include <string>int main() {    // Create a ZeroMQ context and a REQ socket    zmq::context_t context(1);    zmq::socket_t socket(context, zmq::socket_type::req);    socket.connect("tcp://127.0.0.1:55555");  // Ensure this matches GNU Radio's address    std::string msg_content = "Hello World";    // Create a ZeroMQ message with the content    zmq::message_t request(msg_content.size());    memcpy(request.data(), msg_content.c_str(), msg_content.size());    // Send the message to GNU Radio    socket.send(request, zmq::send_flags::none);    // Receive the reply from GNU Radio (required for REQ socket)    zmq::message_t reply;    socket.recv(reply);    return 0;}

The GnuRadio is only 2 blocks: ZMQ REQ Message Source connected to a Message Debug block like the 2 blocks at the bottom of figure 1 here https://wiki.gnuradio.org/index.php/ZMQ_REP_Message_Sink

In ZMQ REQ Message Source I set tcp://127.0.0.1:55555

I did a TCP dump to make sure my C++ sends something, and I can see the "Hello World" strings.

I also tried to send TCP messages directly from Ubuntu: echo "Hello, World!" | nc 127.0.0.1 55555, and I still see them in the TCP dump, but still nothing in GR.

The only way I see the strings in GnuRadio is if I create another flowgraph with ZMQ REP Message Sink sending to the first flowgraph