Is it obligatory to re-fast for someone who masturbated in Ramadan, while ignorant that it invalidates the fast, and uncertain whether it occurred before dawn?

As long as you are doubtful about having masturbated during the daytime in Ramadan, the default is that your fast is valid. If you did it out of ignorance of its prohibition and its being among the things that break the fast, then your fast is not invalidated. However, if you were aware of its prohibition but ignorant of its being among the things that break the fast, then your fast is invalidated, and you must make it up.