Another cause of the issue is sync timing. Office 365 itself is usually not the root of the problem - the mail clients (ActiveSync, iOS devices, iCal) and their connectivity protocol tend to be the culprits. For more information see: Microsoft Calendar Appointment Corruption Issues (August/2018 Update) and Exchange Outlook Calendaring Problems. This issue has been reported in organizations with Office 365, Microsoft Exchange, Outlook (2010, 2013 or 2016), Apple iPhones/iPads, Active Sync devices, Macs running Outlook for Mac, etc. If you've noticed missing calendar appointments, e.g., lost meetings, delegate issues, or meeting updates from someone other than the original organizer, you're not alone.