Ping not being sent from Tomcat WebSocket

Question

Right now, I'm running a very bare bones WebSocket on Tomcat 7 with just a heartbeat in place. It sends a ping to every session every 30 seconds and closes the session after 3 missed pongs. The problem that I'm having is that the server properly sends the first ping but does not send any afterwards. The code for sending the actual ping is getting executed, but from analyzing the network, the ping is not being sent. It also is not throwing any exceptions. Also, I was running this on Tomcat 9 previously and it was not having this problem.

Here is the code for the heartbeat

import java.io.IOException;
import java.nio.ByteBuffer;
import java.util.Arrays;
import java.util.HashMap;
import java.util.Map;
import java.util.concurrent.Executors;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.ScheduledFuture;
import java.util.concurrent.TimeUnit;

import javax.websocket.PongMessage;
import javax.websocket.Session;

public class HeartbeatManager {

    private static final ByteBuffer DATA = ByteBuffer.wrap("heartbeat".getBytes());
    private static final long HEARTBEAT_DELAY_MS = 30 * 1000;
    private static final long MISSED_PONG_LIMIT = 3;

    private ScheduledExecutorService executor = Executors.newScheduledThreadPool(Runtime.getRuntime().availableProcessors());
    private Map<String, Heartbeat> sessionHeartbeats = new HashMap<String, Heartbeat>();

    public void start(Session session) {
        Heartbeat heartbeat = new Heartbeat(session);
        heartbeat.future = executor.scheduleAtFixedRate(heartbeat, HEARTBEAT_DELAY_MS, HEARTBEAT_DELAY_MS, TimeUnit.MILLISECONDS);
        Heartbeat oldHeartbeat = sessionHeartbeats.put(session.getId(), heartbeat);
        if (oldHeartbeat != null) {
            oldHeartbeat.future.cancel(true);
        }
    }

    public void stop(Session session) {
        Heartbeat heartbeat = sessionHeartbeats.remove(session.getId());
        if (heartbeat != null) {
            heartbeat.future.cancel(true);
        }
    }

    public void onPong(Session session, PongMessage message) {
        Heartbeat heartbeat = sessionHeartbeats.get(session.getId());
        if (heartbeat != null) {
            heartbeat.onPong(message);
        }
    }

    private class Heartbeat implements Runnable {

        private final Object LOCK = new Object();

        private ScheduledFuture<?> future;
        private Session session;
        private int missedPongs;
        private boolean pong = true;

        public Heartbeat(Session session) {
            this.session = session;
        }

        public void onPong(PongMessage message) {
            if (Arrays.equals(DATA.array(), message.getApplicationData().array())) {
                synchronized (LOCK) {
                    pong = true;
                    missedPongs = 0;
                }
            }
        }

        @Override
        public void run() {
            synchronized (LOCK) {
                if (!pong && ++missedPongs >= MISSED_PONG_LIMIT) {
                    try {
                        session.close();
                    } catch (IOException e) {
                    }
                } else {
                    pong = false;
                    try {
                        session.getBasicRemote().sendPing(DATA);
                    } catch (IOException e) {
                    }
                }
            }
        }

    }

}

And if it helps, here is the ServerEndpoint

import javax.websocket.OnClose;
import javax.websocket.OnMessage;
import javax.websocket.OnOpen;
import javax.websocket.PongMessage;
import javax.websocket.Session;
import javax.websocket.server.ServerEndpoint;

@ServerEndpoint(value = "/test")
public class Test {

    private static HeartbeatManager m = new HeartbeatManager();

    @OnOpen
    public void onOpen(Session session) {
        m.start(session);
    }

    @OnMessage
    public void onMessage(Session session, PongMessage message) {
        m.onPong(session, message);
    }

    @OnClose
    public void onClose(Session session) {
        m.stop(session);
    }

}