Does the fast of one who masturbated in Ramadan become invalid if she is not certain about the emission of a fluid that necessitates ghusl?

If masturbation leads to the emission of semen, then the fast is broken thereby. If you are not certain of its emission, then qada' (making up the day) is not obligatory, and repentance suffices. As for being ignorant that it is among the things that break the fast, the ruling has already been explained in Fatwa no. 109440.