CsvHelper not writing anything to memory stream

You already have a using block which is great. That will flush your writer for you. You can just change your code slightly for it to work.

using (var memoryStream = new MemoryStream())
{
    using (var streamWriter = new StreamWriter(memoryStream))
    using (var csvWriter = new CsvWriter(streamWriter))
    {
        csvWriter.WriteRecords<T>(records);
    } // StreamWriter gets flushed here.

    return memoryStream.ToArray();
}

If you turn AutoFlush on, you need to be careful. This will flush after every write. If your stream is a network stream and over the wire, it will be very slow.