What? That is not what I have experienced at all. All my named save games are there. Only the auto save and quick save slots are overwritten. All the named save games I have created during play are there no matter from which game. Is this not the case for you? Do you really lose all your named save games when you start a new game?
FYI: I am playing the Windows version (Steam) on Window 7.1. Also, I have only created one profile in the game.