seeking clarity regarding visualization markers publishing

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This is my main function -

int main(int argc, char** argv) {

    ros::init(argc, argv, "explore_node");

    ros::NodeHandle nh("explore_node"); 

    //ros::NodeHandle nh{};

    tf2_ros::Buffer buffer(ros::Duration(10));
    tf2_ros::TransformListener tf(buffer);

    string global_frame, robot_base_frame;  

    nh.param("global_costmap/global_frame", global_frame, std::string("map"));
    nh.param("global_costmap/robot_base_frame", robot_base_frame, std::string("base_link"));

    costmap_2d::Costmap2DROS *global_costmap = new costmap_2d::Costmap2DROS("global_costmap", buffer);    
    costmap_2d::Costmap2DROS *local_costmap = new costmap_2d::Costmap2DROS("local_costmap", buffer);


    costmap_2d::Costmap2D* global_costmap_ = temp_global_costmap_ =  global_costmap->getCostmap();  


    unsigned char* global_og = global_costmap_->getCharMap();

   geometry_msgs::PoseStamped global_pose; 

    bool flag = global_costmap->getRobotPose(global_pose); 

    if(flag) {cout << "global pose found successfully!" << endl;}

    double wx = global_pose.pose.position.x , wy = global_pose.pose.position.y;

    publish_markers(nh, global_costmap_,  wx, wy);  // publish markers function


    return 0;
}

This is how publish_markers function is defined -

void publish_markers(ros::NodeHandle &nh, costmap_2d::Costmap2D* &global_costmap_, double wx, double wy) {

    visualization_msgs::Marker marker;

    marker.header.frame_id = "map";
    marker.header.stamp = ros::Time();
    marker.ns = nh.getNamespace();

    marker.id = 1;
    marker.type = visualization_msgs::Marker::SPHERE;
    marker.action = visualization_msgs::Marker::ADD;


    marker.pose.position.x = wx + 1;
    marker.pose.position.y = wy + 3;
    marker.pose.position.z = 1; 
    marker.pose.orientation.x = 0.0;
    marker.pose.orientation.y = 0.0;
    marker.pose.orientation.z = 0.0;
    marker.pose.orientation.w = 1.0;

    marker.lifetime = ros::Duration(5.0);

    marker.scale.x = 1;
    marker.scale.y = 1;
    marker.scale.z = 1.0;

    marker.color.a = 1.0; // Don't forget to set the alpha!

    marker.color.r = 1.0;
    marker.color.g = 0.0;
    marker.color.b = 0.0;

    ros::Time start = ros::Time::now(); 
    ros::Duration del(30.0);
    ros::Time end = start + del ;

    while(ros::Time::now() < start + del) {

        ros::Publisher vis_pub = nh.advertise<visualization_msgs::Marker>( "visualization_markers", 10 );
        vis_pub.publish(marker);

    }
}

Here's the problem -

The above program doesn't produce the desired marker in Rviz for all small values of del and marker.lifetime. If marker.lifetime is small (say, less than 5), then (roughly speaking) it only produces markers in Rviz for values of del > 20. Why is this happening?