If contacts attempt to update their subscription preferences in the Preferences Center, these updates might not be saved. With this active issue, it is only possible for contacts to opt in or out of a marketing category through the subscribe and unsubscribe hyperlinks inside an email template.
A hotfix is currently being created to resolve the issue, and this article will be updated once the hotfix has been created.
When available, the hotfix should be installed for all Sitecore 10.1 environments that will be using EXM.