Does the act of masturbating during the daytime in Ramadan—while being unaware that it is among the things that invalidate the fast, forgetting this during the act, and then returning to it despite seeking forgiveness and resolving not to return—break the fast and necessitate expiation?

If nothing is discharged from you when practicing this act, then your fast is valid. However, if semen is discharged from you, then your fast is invalidated, and you must make up that day, as long as you know the impermissibility of this act.

You must repent to Allah and desist from this act, both in Ramadan and at other times.