Saying “always” based on your experience with a word that is over 2000 years old without knowing the history is a great way to be wrong. In this case, if there is a group of multiple people, the correct word is alumni, unless the entire group is female, when the word becomes alumnae. Alumni is correct even for a hypothetical group with a billion women and 1 man. If there is just 1 person, it is alumnus for a male and alumna for a female. Most universities would use alumni because there is always 1 male in the group and they want to use the plural. A women’s only university would use alumnae. That said, English speakers have a tendency to mispronounce alumni as alumnae, so trying to maintain a minimum understanding of how to use the word correctly might be a losing battle.