I had the same issue. And now, thanks to your suggestion it works. I switched from:

				m\_toolTip.AddTool(this,ttwtext.c\_str(),cr,1);

To

				TOOLINFO ti;

				ti.cbSize = sizeof(TOOLINFO);
				ti.lpszText = (LPTSTR)last\_ttwtext.c\_str();
				ti.hinst = AfxGetInstanceHandle();
				ti.hwnd = this->GetSafeHwnd();
				ti.uFlags = TTF\_SUBCLASS | TTF\_IDISHWND;
				ti.uId = (UINT) this->GetSafeHwnd();

				m\_toolTip.SendMessage(TTM\_ADDTOOL, 0, (LPARAM) &ti);

Thanks!