"Anonymous Guest" user appearing to be viewing notebook

Follow up post-support call and with a great deal of thanks to the product team.
Firstly, all of the access was set up correctly.  We control access via EntraID security groups and there were only 12 people who could have accessed it - no sharing or links.
Support confirmed that only 2 people (me and the other user) had accessed the notebook.

Another piece of anecdotal evidence was that the other user logged out for the day and exactly at this point the Anonymous Guest appeared.

The cause is down to when a user has ran a job and the left the session, the tooltip that shows who is in a notebook with you no longer 'knows' who the user is when querying Graph API.  So it displays 'Anonymous Guest' where maybe 'Can't get user details' might be more accurate - and it has to display something.

[Note:  I've Accepted the Solution so the support ticket response is visible as part of the issue in case others see it.]