It happens if you have been playing a game for a while without any other activity.(browsing, lurking etc.) Unless you open or reload a page on AG, your "last seen' status is not updated and will change to offline on your profile after around 20-25 minutes. Try opening your profile or any other page on AG in a separate tab before making the comment again.
You can always keep some page like forums open in a separate tab and reload it every fifteen or so minutes to avoid being marked as offline.
