cppkafka BufferedProducer::Produce not flushing the queue

Question

#include <stdexcept>
#include <iostream>
#include <boost/program_options.hpp>
#include "cppkafka/utils/buffered_producer.h"
#include "cppkafka/configuration.h"

using std::string;
using std::exception;
using std::getline;
using std::cin;
using std::cout;
using std::endl;

using cppkafka::BufferedProducer;
using cppkafka::Configuration;
using cppkafka::Topic;
using cppkafka::MessageBuilder;
using cppkafka::Message;

namespace po = boost::program_options;

int main(int argc, char* argv[]) {
    string brokers;
    string topic_name;
    int partition_value = -1;

    po::options_description options("Options");
    options.add_options()
        ("help,h",      "produce this help message")
        ("brokers,b",   po::value<string>(&brokers)->required(), 
                        "the kafka broker list")
        ("topic,t",     po::value<string>(&topic_name)->required(),
                        "the topic in which to write to")
        ("partition,p", po::value<int>(&partition_value),
                        "the partition to write into (unassigned if not provided)")
        ;

    po::variables_map vm;

    try {
        po::store(po::command_line_parser(argc, argv).options(options).run(), vm);
        po::notify(vm);
    }
    catch (exception& ex) {
        cout << "Error parsing options: " << ex.what() << endl;
        cout << endl;
        cout << options << endl;
        return 1;
    }

    // Create a message builder for this topic
    MessageBuilder builder(topic_name);

    // Get the partition we want to write to. If no partition is provided, this will be
    // an unassigned one
    if (partition_value != -1) {
        builder.partition(partition_value);
    }

    // Construct the configuration
    Configuration config = {
        { "metadata.broker.list", brokers }
    };

    // Create the producer
    BufferedProducer<string> producer(config);

    // Set a produce success callback
    producer.set_produce_success_callback([](const Message& msg) {
        cout << "Successfully produced message with payload " << msg.get_payload() << endl;
    });
    // Set a produce failure callback
    producer.set_produce_failure_callback([](const Message& msg) {
        cout << "Failed to produce message with payload " << msg.get_payload() << endl;
        // Return false so we stop trying to produce this message
        return false;
    });

    cout << "Producing messages into topic " << topic_name << endl;

    // Now read lines and write them into kafka
    string line;
    while (getline(cin, line)) {
        // Set the payload on this builder
        builder.payload(line);

        producer.produce(builder);

    }

}

I am experiencing an issue with the producer.produce(builder) function in my Kafka producer. The function internally calls rd_kafka_producev, and according to the librdkafka documentation, the internal Kafka queue should be flushed if either queue.buffering.max.messages is exceeded or linger.ms (also known as queue.buffering.max.ms) exceeds the specified time to accumulate up to batch.num.messages.

My current configuration is as follows:

{"linger.ms", 75},
{"queue.buffering.max.messages", 1000}

Theoretically, the internal Kafka queue should be flushed after 75 milliseconds, accumulating up to 1000 messages. However, in my case, the internal kafka queue is not being flushed until a new event is produced/generated/triggered.

I have double-checked the configuration, and there don't seem to be any errors or exceptions during the producer's operation.