Hi David,
Good day to you. Thanks for querying in this forum.
According to your description, you cannot find Harvard style from the References tab.
I did a test in Word Current Channel(Preview), version 2202(Build 14931.20132). It seems like I cannot reproduce the issue.

May I know if the issue happens in a specific document? If you create a new document, does it work? If the new document works, you may try to copy all content from the original document and paste it into the new document.
If different documents all have this issue, very sorry for the inconvenience caused. You may try following troubleshooting methods:
1.Close all documents and then open Word application in safe mode. Then create a new document from File>New, go to References>Style, check the outcome.
If it works in safe mode, the issue should be caused by an add-in in Word.
2.Online Repair Office. For detailed steps, you may refer to this article Repair an Office application (microsoft.com)
If issue persists, we do understand the frustration and apologize for the inconvenience. We appreciate your understanding that sometimes the initial reply may not resolve the problem very soon. However, we can work together to narrow down and resolve the situation. Please kindly provide more information to let us work further.
1.What is the version of your Word app? Please go to File>Account, capture a full screenshot of Product Information.
2.Does the issue happen in a specific documents or different documents?
Thanks for your patience and understanding. Hope you are keeping safe and well.
Best regards,
Tina