Hi Dodd Emma E
Thankyou for your Response.
I'm glad that you were able to resolve your issue and thank you for posting your solution so that others experiencing the same thing can easily reference this! Since the Microsoft Q&A community has a policy that "The question author cannot accept their own answer. They can only accept answers by others ", I'll repost your solution in case you'd like to accept the answer.
Issue: Why are my bookmarks not showing up in a hub search?
We have created several Bookmarks but they are not showing up in our search that is currently limited to a hub and every article
Solution: According to this and other answers I finally found, it is not supported: https://learn.microsoft.com/en-us/answers/questions/1185614/bookmark-support-on-hub-sites.
The suggestion link no longer goes to an actual feedback item, but searching returned this one that was posted 22 days ago which only has 4 votes, which includes mine from yesterday: https://feedbackportal.microsoft.com/feedback/idea/1ed24df0-91a7-ef11-95f5-6045bdbc7c39
If the answer is helpful, please click "Accept Answer" and kindly upvote it.