Why are the properties in my message getting set to null?

When I produce a message to my Kafka topic, I can see the messages with properties assigned in the MSK Kafka UI.

When I print out the serialized message in my function, all of my properties are being set to null:

Received input: 
{
    "Media": null,
    "Season": null,
    "Content": null,
    "Collection": null,
    "Key": null,
    "Id": 0,
    "Type": null,
    "Action": null,
    "MD5": null
}

The following is my code:

using Amazon.Lambda.Core;
using System.Xml.Linq;
using IVITransformer.Models;
using Newtonsoft.Json;




// Assembly attribute to enable the Lambda function's JSON input to be converted into a .NET class.
//[assembly: LambdaSerializer(typeof(Amazon.Lambda.Serialization.SystemTextJson.DefaultLambdaJsonSerializer))]
[assembly: LambdaSerializer(typeof(Amazon.Lambda.Serialization.SystemTextJson.DefaultLambdaJsonSerializer))]

namespace AWSLambdaIVITransformer;

public class Function
{
    
    /// <summary>
    /// A simple function that takes a string and does a ToUpper
    /// </summary>
    /// <param name="input">The event for the Lambda function handler to process.</param>
    /// <param name="context">The ILambdaContext that provides methods for logging and describing the Lambda environment.</param>
    /// <returns></returns>
    public string FunctionHandler(MessageModel input, ILambdaContext context)
    {
       
        Console.WriteLine("Received input: " + JsonConvert.SerializeObject(input));
        if (input == null)
        {
            Console.WriteLine("No input provided");
            return "No input provided";
        }
        else if (input.Content == null)
        {
            Console.WriteLine("No content provided");
            return "No content provided";
        } 
        else if (input.Content.ContentId == null )
        {
            Console.WriteLine("Invalid content id provided");
            return "Invalid content id provided";

        }


        Console.WriteLine("Received inputt: " + input.Content.ContentId.ToString());
        return input.Content.ContentId.ToString();

    }
}

My code works when using the AWS provided test harness (provided by the Lambda Visual Studio project type). I’m able to print out a Content ID. Does anyone know why the values are seemingly being stripped?