Does one have to make up for fasting the sixth day, which did not coincide with menstruation, or does it fall under the ruling of intentional breaking of the fast in Ramadan?

You must fast today, because some scholars hold the view that if a woman sees dryness during her period, she may wait for a day and a half after seeing the dryness. They justified this by stating that the nature of blood is flow and cessation. This is the preferred view of Ibn Qudamah and Sheikh Al-Uthaymeen. You have already waited a full day, and there is no blame on you. The ruling on one who breaks their fast in Ramadan without an excuse does not apply to you.