Redirected output from console application is VERY inconsistent

Question

I've got a console application (steamcmd.exe) which I want to redirect its output to a textbox in my Winforms application.

https://steamcdn-a.akamaihd.net/client/installer/steamcmd.zip

I've managed to do that, but the output I get is very inconsistent. For example, I start steamcmd.exe normally with these arguments

+login anonymous +force_install_dir ./unturned/ +app_update 1110390 validate

This will install the Unturned game server. The output is perfectly fine. It shows the downloading progress consistently.

But when I start steamcmd.exe using my Winforms application and I redirect its output, it becomes super inconsistent, it could stay stuck at 2% and after a few minutes jump to 58%, the get stuck again and jump to 88% etc.

Here is my code:

   steamCMDProc = new Process();
   void startSteamCMD()
    {
        steamCMDProc.EnableRaisingEvents = true;
        steamCMDProc.OutputDataReceived += new DataReceivedEventHandler(steamCMD_OutputDataReceived);
        steamCMDProc.ErrorDataReceived += new DataReceivedEventHandler(steamCMD_ErrorDataReceived);
        steamCMDProc.Exited += new System.EventHandler(steamCMDProcExited);

        steamCMDProc.StartInfo.FileName = srvInstDir + @"\steamcmd.exe";
        steamCMDProc.StartInfo.Arguments = "login anonymous +force_install_dir ./unturned/ +app_update 1110390 validate";
        steamCMDProc.StartInfo.UseShellExecute = false;
        steamCMDProc.StartInfo.RedirectStandardError = true;
        steamCMDProc.StartInfo.RedirectStandardOutput = true;
        steamCMDProc.StartInfo.RedirectStandardInput = true;
        steamCMDProc.StartInfo.ErrorDialog = false;
        steamCMDProc.StartInfo.CreateNoWindow = true;
        steamCMDProc.StartInfo.WindowStyle = ProcessWindowStyle.Hidden;
       
        steamCMDProc.Start(); 
       
        // inputWriter = steamCMDProc.StandardInput; not currently used
        StartThread(steamCMDProcExited); //i start a thread which constantly loops and waits till steamCMD has exited
        steamCMDProc.BeginErrorReadLine();
        steamCMDProc.BeginOutputReadLine();
   }
   void steamCMD_ErrorDataReceived(object sender, DataReceivedEventArgs e)
   {
         InvokeIfRequired(textBox2, () => //function that checks if invoke is required
         { textBox2.AppendText(e.Data + Environment.NewLine); });
    }

    void steamCMD_OutputDataReceived(object sender, DataReceivedEventArgs e)
    {      
         InvokeIfRequired(textBox2, () =>
         { textBox2.AppendText(e.Data + Environment.NewLine); });
    }