Dear Dean_063
Hope this email finds you well.
Apologies for the late update as we were checking further into this issue if it's a bug or not. Based on our test on different environments, we were still able to add the agenda. Our online team confirm this to be a bug and request to involve them to get on remote session with you to fix the issue from backend as they have higher permission compared to us in Microsoft forum community.
If you have the organization admin permission, you can raise a service request to report this symptom to our online channel through the O365 admin center. A support agent can further connect with you remotely, for further troubleshooting and involve the backend team where need be. Get support for Microsoft 365 for business -- Online support. If you need guidance on how to find your admin, see How do I find my Microsoft 365 admin? - Microsoft Support
I will also revert back when I get more info on this. Your patience and cooperation are much appreciated.
Have a great day.