Is it the case that if one intended to fast a day in fulfillment of a missed Ramadan fast due to a legitimate excuse, but then broke their fast on that day, they are obligated to make up two days: one day for the missed Ramadan fast, and another day for the makeup fast that they broke?

A Muslim is not permitted to break their fast while making up for Ramadan or any other obligatory fast, except if there is a legitimate Shar’i excuse that permits them to break it. Whoever does so commits a sin and must repent to Allah. As for one who breaks their fast due to an excuse, there is no sin upon them. In both cases, whoever breaks their fast is only required to fast one day.