Instead, in order to allow Office 365 meetings to work with other calendaring systems and external users, all calendaring data requires the sending and receiving of email messages to all attendees. For instance, if someone accepts a meeting at 9am on an iPhone and a delegate declines it at 9:10am on a PC, but the iPhone is out of signal range and doesn't sync the meeting until say 9:30am., what happens to the meeting? Many people assume that the Office 365 calendar system tracks a meeting as a single copy and that any edits to a meeting will automatically update and appear for all attendees.
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.
