SerialPort BaseStream.EndRead return data even after disconnecting the device

Question

I have a device that is connected to the computer with a USB cable (Prolific USB to Serial ), I use the following code to read data

(code taken from https://www.vgies.com/a-reliable-serial-port-in-c/)

       public class ReliableSerialPort : SerialPort
       {
       :
       :
         private void ContinuousRead()
         {
          byte[] buffer = new byte[4096];
          Action kickoffRead = null;
          kickoffRead = () =>
          {
             try
             {
                 BaseStream.BeginRead(buffer, 0, buffer.Length, delegate (IAsyncResult ar)
                {
                    try
                    {
                        int count = BaseStream.EndRead(ar);
                        byte[] dst = new byte[count];
                        Buffer.BlockCopy(buffer, 0, dst, 0, count);
                        OnDataReceived(dst);
                    }
                    catch (Exception exception)
                    {
                        Console.WriteLine($"OptimizedSerialPort exception!\r\n {exception.ToString()}");
                    }

                    kickoffRead();
                }, null);
            }
            catch (Exception e)
            {
                //When disconnecting while accessing BaseStream an InvalidOperationException is raised, we ignore it
            }
        };

        kickoffRead();
       
    }

Every few seconds I also send a message to the device.

After disconnecting the cable from the device (the cable stays connected to the computer) I still get data!!!

i.e. the line int count = BaseStream.EndRead (ar); Returns a number greater than 0.

the data is the message I sent.

What could be the reason?