Hello Zaphod.Beeblebrox.III
Good day and thanks for posting in Microsoft community
Restoring a declined meeting invite that was part of a series in Outlook can be a bit complex. Can you please confirm if you have checked for the declined meeting in Deleted Items folder of your Outlook account? If you declined the invitation, the original meeting request may still be accessible here. Search for the series name or the meeting organizer’s name, and if you find the original invite, you can select it and choose to accept it again. Show a declined meeting on my calendar - Microsoft Support
However, given that the issue affects all platforms; we will recommend you reached out to Microsoft back-end for advanced troubleshooting. They specialize in advanced troubleshooting techniques, tools and have the expertise necessary to delve deeper into the root cause of the issue. Microsoft's back-end support team should collect some advanced log information from Office 365 Global Admin personnel through a remote session to diagnose the current scenario.
Please keep in mind as a forum supporter, we do not have sufficient permissions and resources to conduct research directly. We recommend that you contact your Microsoft 365 administrator and follow the steps in this article Get support - Microsoft 365 admin | Microsoft Learn **** to create a support ticket to Microsoft Teams to investigate further.
You can also reach Microsoft back-end support via- Contact - Microsoft Support
Note: If you are an end-user person, you may need to contact the Office 365 Global Administrator in your business organization to contact a Technical Support Engineer for further processing.
Important: If any of your organization's Office 365 Business/Business/Education subscriptions are from a federated partner or reseller and the global admin can't open a service request on their end, they may need to contact the reseller's support provider so they can help the global admin open the service request on their end. After that, the Office 365 support team will participate in the service request that was created.
Thank you very much for your valuable time in your cooperation.
Sincerely,
Bertrand