微信小程序解密偶发性的 "填充无效，无法被移除"

C#基础代码片段：

private static byte[] AES_decrypt(string input, string iv, string key)
        {
            RijndaelManaged aes = new RijndaelManaged();
            aes.KeySize = 256;
            aes.BlockSize = 128;
            aes.Mode = CipherMode.CBC;
            aes.Padding =  PaddingMode.PKCS7;
            aes.Key = Convert.FromBase64String(key);
            aes.IV = Convert.FromBase64String(iv);
            var decrypt = aes.CreateDecryptor(aes.Key, aes.IV);
            byte[] xBuff = null;
            using (var ms = new MemoryStream())
            {
                using (var cs = new CryptoStream(ms, decrypt, CryptoStreamMode.Write))
                {
                    byte[] xXml = Convert.FromBase64String(input);
                     byte[] msg = new byte[xXml.Length + 32 - xXml.Length % 32];
                     Array.Copy(xXml, msg, xXml.Length);
                    cs.Write(xXml, 0, xXml.Length);
                }
                xBuff = decode2(ms.ToArray());
               
            }
            return xBuff;
        }       

 private static byte[] decode2(byte[] decrypted)
        {
            int pad = (int)decrypted[decrypted.Length - 1];
            if (pad < 1 || pad > 32)
            {
                pad = 0;
            }
            byte[] res = new byte[decrypted.Length - pad];
            Array.Copy(decrypted, 0, res, 0, decrypted.Length - pad);
            return res;
        }

错误日志：

ExecptionMessage:Padding is invalid and cannot be removed.

mscorlib

   at System.Security.Cryptography.RijndaelManagedTransform.DecryptData(Byte[] inputBuffer, Int32 inputOffset, Int32 inputCount, Byte[]& outputBuffer, Int32 outputOffset, PaddingMode paddingMode, Boolean fLast)

   at System.Security.Cryptography.RijndaelManagedTransform.TransformFinalBlock(Byte[] inputBuffer, Int32 inputOffset, Int32 inputCount)

   at System.Security.Cryptography.CryptoStream.FlushFinalBlock()

   at System.Security.Cryptography.CryptoStream.Dispose(Boolean disposing)

   at System.IO.Stream.Close()

   at System.IO.Stream.Dispose()

   at Senparc.Weixin.WxOpen.Helpers.EncryptHelper.AES_Decrypt(String Input, Byte[] Iv, Byte[] Key)

   at Senparc.Weixin.WxOpen.Helpers.EncryptHelper.DecodeEncryptedData(String sessionKey, String encryptedData, String iv)

加密数据：

sessionKey:98195476102492321891401391061935624977242；IV:116115241129461711788323441202601974169239；

encryptedData:deWfUALVVTxrux2cp0qeqLWotTHTIpRmIrpcuWoh3ngyr7vjCDYq1wh2Q0CE6Zj9P/V2ZVqtjkVAiGdBuBR8fSs9qpWhb9ieO5FoumuvgoM6HP5+7Eul6lm8njXJlbTZr+pODAIeMoBIwpQUPpCLwYtpSuKlQGKvsrmoVU5j5xgoKm4dyKmNwq3qcqE5Q+HUOV0r/c7GusFWZD0haaccduMjmKAyupCpbwdDu6kiVfEo1pVZdp5j4C5ihrZdE7gzeS9vOAFDaB+NXPB6Lz+H8js6BH8gVJ7tZ1KUAwqt+FIqHHBKsREKoyjePwREkRc1Sr/N+QR1vps2cFGpqp16NAoTyT/JFi2jNs8PgrrEYZkjVvyMUYFlDnq5BWNyyh5RX34JEq7EN62sc+wfAMB2Nrm/QEcBCtYLycP3xcQnCLasU2SQbpIr5GOUz7aiIu5rwMXMUDDg7jxCOA4+ORfSHUgS6OczRjY+QqrcfKmlA84=