fileapi::WriteFile() doesn't send an input, if processthreadsapi::STARTUPINFO::hStdError is set (ffmpeg)

I'm trying to capture my screen using ffmpeg in a different thread (which I create using processthreadsapi::CreateProcess()) so I'd be able to do something else in the main thread, and redirect ffmpeg output, so it wouldn't pop up in the console for the user to see. To stop filming, I send a 'q' input using WriteFile(), and after that I want to save ffmpeg accumulated output using ReadFile().

However, if I set STARTUPINFO::hStdError (note, that ffmpeg output goes to stderr) to a pipe, from which I could read the accumulated data, the inputs I send using WriteFile() are no longer registered and ffmpeg.exe keeps running.

I've tried redirecting ffmpeg output in a simple command line, but I can still stop the process by pressing the q button.

Also, if I record for less than 8 seconds, the input is registered and ffmpeg.exe closes.

Is there something wrong with my code, or is it processthreadsapi issue, any hints will be kindly appreciared!

Here's a minimal code of how I am trying to do it:

#include <iostream>

#include <conio.h>
#include <windows.h>

using namespace std;

HANDLE g_hChildStd_IN_Rd = NULL;
HANDLE g_hChildStd_IN_Wr = NULL;
HANDLE g_hChildStd_OUT_Rd = NULL;
HANDLE g_hChildStd_OUT_Wr = NULL;

int main()
{
    //Create IN and OUT pipes
    SECURITY_ATTRIBUTES saAttr;
    saAttr.nLength = sizeof(SECURITY_ATTRIBUTES);
    saAttr.lpSecurityDescriptor = NULL;


    if (! CreatePipe(&g_hChildStd_OUT_Rd, &g_hChildStd_OUT_Wr, &saAttr, 0) )
        cout<<"StdoutRd CreatePipe error"<<endl;

    if (! CreatePipe(&g_hChildStd_IN_Rd, &g_hChildStd_IN_Wr, &saAttr, 0) )
        cout<<"Stdin CreatePipe error"<<endl;


   PROCESS_INFORMATION piProcInfo;
   STARTUPINFO siStartInfo;

   ZeroMemory( &piProcInfo, sizeof(PROCESS_INFORMATION) );
   ZeroMemory( &siStartInfo, sizeof(STARTUPINFO) );

   siStartInfo.cb = sizeof(STARTUPINFO);
   siStartInfo.hStdError = g_hChildStd_OUT_Wr;
   siStartInfo.hStdOutput = g_hChildStd_OUT_Wr;
   siStartInfo.hStdInput = g_hChildStd_IN_Rd;
   siStartInfo.dwFlags |= STARTF_USESTDHANDLES;


   //Start recording
   if(!CreateProcess(NULL,
      "ffmpeg -y -f gdigrab -framerate 2 -i desktop record.avi",     // command line
      NULL,          // process security attributes
      NULL,          // primary thread security attributes
      TRUE,          // handles are inherited
      0,             // creation flags
      NULL,          // use parent's environment
      NULL,          // use parent's current directory
      &siStartInfo,  // STARTUPINFO pointer
      &piProcInfo))  // receives PROCESS_INFORMATION
   {
    cout<<"Error create process"<<endl;
   }
   else
   {
       cout<<"Process created successfully"<<endl;
       cout<<"Press k to stop recording"<<endl;
   }

    //Record for a while
    while(getch() != 'k'){
        cout<<"While press k"<<endl;
    };

   cout<<"Stop recording called"<<endl;

    //Stop recording by emulating a Q button push
    DWORD dwWritten;
    CHAR chBufW[1] = {'q'};

    if ( ! WriteFile(g_hChildStd_IN_Wr, chBufW, 1, &dwWritten, NULL) )
        cout<<"Error write file"<<endl;

    //Save stdError (ffmpeg) data
    DWORD dwRead;
    char stdErrorData[4096];
    bool bSuccess;

    bSuccess = ReadFile( g_hChildStd_OUT_Wr, stdErrorData, 4096, &dwRead, NULL);

    if(!bSuccess || dwRead == 0)
        cout<<"Read failed"<<endl;
    else{
        cout<<"Read success"<<endl;
    }

    cout<<"Press to exit"<<endl;
    while(getch() != 'k');
    return 0;
}