apple apns notifications are being delivered out of sequence. one previous push notification is being sent instead of current one

Question

I have configured my push notifications using the node apn library.

I use an AWS lambda function to send the notification and it is called from another lambda function using event as the invocation type as follows:

var pushPayload = { "users": users, "message": message };

var pushParams = {
    FunctionName: 'function-name',
    InvocationType: 'Event',
    LogType: 'Tail',
    Payload: JSON.stringify(pushPayload)
};

lambda.invoke(pushParams, function (err, data) {
    if (err) {
        callback(error);
    } else {
        callback(null, event.arguments.input);
    }
});

and the send notification lambda function:

var options = {
    token: {
        key: "key",
        keyId: keyId,
        teamId: teamId
    },
    production: true
};
var message = event.message;
var users = event.users;

var apnProvider = new apn.Provider(options);

var iterationComplete = false;

for (var j = 0; j < users.length; j++) {
    if (j === (users.length - 1)) {
        iterationComplete = true;
    }

    var deviceToken = users[j].user_device_token;

    var notification = new apn.Notification();

    notification.alert = message;

    notification.contentAvailable = 1;

    notification.topic = "com.example.Example";

    context.callbackWaitsForEmptyEventLoop = false;
    apnProvider.send(notification, [deviceToken]).then((response) => {
        if (iterationComplete) {
            context.succeed(event);
        }
    });
}

Sometimes the notifications get out of sync and when I send a message which sends it's push notification, the notification from the previous message is received. Not sure why this is happening or how to fix it.