I'm using Form timer fired by serialport datareceived. Sometimes timer should stopped by user but timer doesn't stop but it doesn't

Here is my summarized code.

private delegate void MessageDelegate(string str);
private MessageDelegate addmsg;
private void SerialPort_DataReceived(object sender, System.IO.Ports.SerialDataReceivedEventArgs e)
{
  if (!StopRequests_DataReceivedRunning) //initially false and set true by StopMenuItem_Click()
    {
       addmsg = new MessageDelegate(InterpretMessage);
       //...get seriaport string...
       Invoke(addmsg, new object[] { strDataReceived.ToString() });
     }
}
private void InterpretMessage(string ReceivedMessage)
{
  //...do something
  if(condition is met)
    {
      timer.Interval = 10000
      timer.Start();
    }

}
private void timer_Tick(object sender, EventArgs e)
{
  Console.WriteLine(timer.Enabled.ToString()); ;
  //<do-work>
}
private void StopMenuItem_Click(object sender, EventArgs e)
{
  timer.Enabled = false;
  StopRequests_DataReceivedRunning = true;
}

If I click StopMenuItem during running of timer, it doesn't stop and finally raise timer_Tick event(console output is true). Also, it looks timer restart at the moment, timer.Enabled = false.

Edit: I added more codes.