Is it obligatory to make up for two days of fasting (a make-up day and a missed day) if a person breaks their make-up fast without an excuse? And does the same ruling apply to voluntary fasting?

If you intentionally broke your fast on a day of qada' (making up for a missed Ramadan fast) without a valid excuse, you have committed a sin. You must repent to Allah the Almighty, and according to most scholars, you must make up for that spoiled day with one day. As for voluntary (nafl) fasting, there is no sin in intentionally breaking it, because "the volunteer is his own master," except that the Malikis and Hanafis obligate the completion of a voluntary fast and obligate its make-up if the fasting person intentionally nullifies it.