How to post separate messages to Kinesis

Hi, I'm using a Kinesis + Firehose to write the received messages to S3 with each message represented as a single file. However, I found that all messages I write to Kineses are get serialised in a single file, instead of separate files. What could be causing this behaviour?

Context: I'm using .NET client to generate and write JSON messages to Kinesis

Example:

// post JSON messages from jsonObjects array
foreach(var jsonMessage in jsonObjects)
{
    var postMessage = PutKinesisData(awsClient, jsonMessage);
}

PutRecordResponse PutKinesisData(AmazonKinesisClient client, JObject jsonObject)
{
    var jsonString = JsonConvert.SerializeObject(jsonObject);
    byte[] messageObj = Encoding.UTF8.GetBytes(jsonString);

    var request = new PutRecordRequest();
    request.StreamName = KINESIS_STREAM_NAME;
    request.PartitionKey = PARTITION_KEY;
    request.Data = new MemoryStream(messageObj);

    var response = client.PutRecordAsync(request).Result;
    return response;
}

As per documentation, the PutRecord was supposed to post single messages, but for some reason, I'm ending up with a single file in S3 that contains all my JSON messages combined

WHY IS IT THE ISSUE: as per current behaviour my array of JSON is being written to the single S3 file as {json1}{json2} which makes the JSON file incorrect, instead of separate JSON files