"Anonymous Guest" user appearing to be viewing notebook

Hi spencer_sa ,
Just checking in to see if you had a chance to raise the support ticket we talked about earlier. Let me know if you’ve already done it or if you need any help with the process.

Happy to assist if needed!
https://learn.microsoft.com/en-us/power-bi/support/create-support-ticket

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We appreciate your engagement and thank you for being an active part of the community.

Best regards,
LakshmiNarayana

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.]