A family of Microsoft word processing software products for creating web, email, and print documents.
The most likely reason it keeps vanishing is how the list was created. There are two similar commands and only one of them saves a reusable style. If you used Define New Multilevel List, the numbering belongs to that document only. It will not come back in a new document. Use Define New List Style instead. Go to Home, then Multilevel List, then Define New List Style. Name it Harvard Outline. Set each level the way you want it, I, then A, then 1, then a. Tick the option for new documents based on this template. Then click OK. Next, watch what happens when you quit Word. If Word asks whether to save changes to Normal.dotm, choose Save. If you choose Do Not Save, the work is thrown away and the style is gone again. That prompt is an easy one to miss. To check whether it worked, open a new blank document. Go to Home, then Multilevel List, and look under List Styles for Harvard Outline. If it is not there, it never reached Normal.dotm. One more point if you use the Organizer. Copy the list style and the paragraph styles linked to it. Copying the list style alone is not enough.
This answer was prepared with help from Muse.