I am using C# to decrypt a request from data_exchange in Whatsapp flows successfully using BouncyCastle 2.3.1,
however I am having issues on decryption whatsApp playground says it cannot decrypt the response that I am providing(from the server).
Below is my code.

var aesKey = DecryptUsingRsa(privateKey, encryptedAesKey); //Decrypting the aes_key from the request and its working on decryption.

Working Decryption method

public static byte[] Encrypt(byte[] plaintext, byte[] key, byte[] iv)
{
    var cipher = new GcmBlockCipher(new AesEngine());
    var parameters = new AeadParameters(new KeyParameter(key), 128, iv);
    cipher.Init(true, parameters);

    var ciphertext = new byte[cipher.GetOutputSize(plaintext.Length)];
    var len = cipher.ProcessBytes(plaintext, 0, plaintext.Length, ciphertext, 0);
    cipher.DoFinal(ciphertext, len);

    var authenticationTag = cipher.GetMac();

    var combinedCiphertext = new byte[ciphertext.Length + authenticationTag.Length];
    Array.Copy(ciphertext, 0, combinedCiphertext, 0, ciphertext.Length);
    Array.Copy(authenticationTag, 0, combinedCiphertext, ciphertext.Length, authenticationTag.Length);

    return combinedCiphertext;

}//Working decryption method

The WAExchangeResponse is the response to be sent to WhatsApp according to the expected structure

public class WAExchangeResponse
{
    public string version { get; set; }
    public string screen { get; set; }
    public Dictionary<string, string> data { get; set; }
}

byte[] responsePayload = Encoding.UTF8.GetBytes(JsonSerializer.Serialize(wAExchangeResponse));// convert the response object to byte array after I serialize it.

byte[] flippedIV = FlipIv(initialVector);// byte array with inverted bits from the  initialization vector received in the request.

    private static byte[] FlipIv(byte[] iv)
    {
        for (int i = 0; i < iv.Length; i++)
        {
            iv[i] ^= 0xFF; // Invert all bits
        }
        return iv;
    }// the method

var encryptedPayload = Encrypt(responsePayload, aesKey, responseIV);//encrypting the payload

    private static byte[] Encrypt(byte[] plaintext, byte[] key, byte[] iv)
    {
        var cipher = new GcmBlockCipher(new AesEngine());
        var parameters = new AeadParameters(new KeyParameter(key), 128, iv);
        cipher.Init(true, parameters);

        var ciphertext = new byte[cipher.GetOutputSize(plaintext.Length)];
        var len = cipher.ProcessBytes(plaintext, 0, plaintext.Length, ciphertext, 0);
        cipher.DoFinal(ciphertext, len);

        var authenticationTag = cipher.GetMac();

        var combinedCiphertext = new byte[ciphertext.Length + authenticationTag.Length];
        Array.Copy(ciphertext, 0, combinedCiphertext, 0, ciphertext.Length);
        Array.Copy(authenticationTag, 0, combinedCiphertext, ciphertext.Length, authenticationTag.Length);//appending authentication tag generated during encryption to the end of the encryption result;

        return combinedCiphertext;

    }

string base64String = Convert.ToBase64String(encryptedPayload);//encoding the whole output as base64 string

Then in the controller I call the servic that return the above base64String as below

    [HttpPost("")]
    public IActionResult Create(WAEndpointRequest wAEndpointRequest)
    {
        var base64String = _processor.Create(wAEndpointRequest);

        Response.ContentType = "text/plain";
        Response.ContentLength = base64String.Length; 
        Response.StatusCode = 200;
        return Content(base64String, "text/plain", Encoding.UTF8);
    }

This is according to the documentation here https://developers.secure.facebook.com/docs/whatsapp/flows/guides/implementingyourflowendpoint#request-decryption-and-encryption

I was expecting Whatsapp playground to be able to decrypt the response I am providing since the decryption is working well and I am following the documentaion.