Hi @frob ,
Per my test, we have to manually add sites in the top navigation.
Edit hub navigation > Add link
For Reference:
Customize the navigation on your SharePoint site
Set up your SharePoint hub site
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.