Callback not being called on first spinOnce()

edit

I am using Melodic on Ubuntu 18.04 and using the turtlesim_node from turtle_sim. The turtlesim_node publishes its pose on the topic of /turtle1/pose. I have a subscriber node that I wrote below:

#include "ros/ros.h"
#include <geometry_msgs/Twist.h>
#include <turtlesim/Pose.h>
#include <math.h>

class TurtleSimPose
{
public:
  TurtleSimPose(ros::NodeHandle& nh)
  {
    pose_subscriber = nh.subscribe<turtlesim::Pose>("/turtle1/pose", 1, 
    &TurtleSimPose::poseCallback, this);
  }

  void poseCallback(const turtlesim::Pose::ConstPtr & pose_message)
  {
    turtlesim_pose.x = pose_message->x;
    turtlesim_pose.y = pose_message->y;
    turtlesim_pose.theta = pose_message->theta;
    ROS_INFO("Callback function called");
  }

  ros::Subscriber pose_subscriber;
  turtlesim::Pose turtlesim_pose;
};

int main(int argc, char **argv)
{
    ros::init(argc, argv, "cleaner_node");
    ros::NodeHandle nh;

    ROS_INFO("Cleaner node has been initialized");
    TurtleSimPose turtleSimPose(nh);

    ros::Rate r(1);
    ros::spinOnce();
    r.sleep();
    ROS_INFO("poseX=%f", (double)turtleSimPose.turtlesim_pose.x);
    ros::spinOnce();
    ROS_INFO("poseX=%f", (double)turtleSimPose.turtlesim_pose.x);
    r.sleep();
}

When I run the node, the output is as follows

[ INFO] [1609025627.687273851]: Cleaner node has been initialized
[ INFO] [1609025637.689161925]: poseX=0.000000
[ INFO] [1609025637.689295814]: Callback function called
[ INFO] [1609025637.689356096]: poseX=5.544445

Why is the callback function not being called during the first spinOnce function call, but for the second function call, the class member variable is changed and the callback function is called. I am confused about this.

Also, when I change the rate to more than 1Hz, I noticed during the first second of the subscriber node being initialized that the callback function is not called, but only after that first second, will be it called.

Thank you in advance