Must a missed Ramadan day be made up if you woke up after the Maghrib Adhan and doubted whether menstruation began after the Adhan or before it?

As long as you are not certain that the blood appeared before sunset, your fast is valid. This is because the original presumption is that you were eligible to fast, and the event (blood) is attributed to the nearest time it could have occurred, which here is after sunset. This is the view held by Sheikh Ibn Uthaymeen.