Does masturbating after Ramadan invalidate the Ramadan fast, knowing that I do not do it during Ramadan?

Your fast is valid and fulfills the obligation, clearing your responsibility, and you are not required to make it up. Your abandonment of sin in Ramadan is a good deed, and it is even better that you make a sincere repentance (tawbah nasuhah) by completely giving up the sin and not returning to it. Sincere repentance is that in which one firmly resolves never to return to the sin at all. Indeed, evil deeds can nullify the reward of some good deeds, so be careful that your sins do not negate the reward of your righteous actions.