Overview
There have been reported and well documented cases where calendar appointments and meetings either disappear, don't sync correctly to mobile devices or aren't added to mobile devices correctly. This can affect anyone using Outlook, using an iPhone or iPad, or an Android device. Our mail system is Microsoft 365 and our supported and preferred mobile devices are Apple's iOS and iPadOS devices including the iPhone and iPad.
Issues can occur when syncing calendar items between mobile devices and Microsoft 365. There are various reasons why these issues occur. Some of them have to do with the device, some with the software being used and how its used and other times the issue is with the appointments themselves. Below is a list of best practices compiled from various articles and Microsoft's documentation to help minimize problems.
General Best Practices
Stay up-to-date
Make sure that all devices that will sync a calendar are up-to-date and all apps versions are current. Apple releases periodic updates to repair many different OS issues and Microsoft releases periodic updates for Outlook on iOS/iPadOS, macOS, and Windows. If you need assistance with updating your device or software, contact the Service Desk.
All devices should stay current
Every computer and app for people that will manage a calendar should be running the same version of Outlook with the same hot-fixes and patches applied. Because not all email applications or patches are the same, any discrepancy between versions could cause complications. The same goes for mobile devices. If a calendar will be appearing on more than once device, then each device should also be running the same version of the software.
This applies not only to multiple people managing a shared calendar, but also to an individual using multiple computers to manage their own calendar.
One active delegate
Only one person should manage all calendar events at a time. It's OK to have more than one person setup to view calendars however it should be decided between those who share the calendar which one person will manage all events and appointments. If more than one person manages the events, Microsoft 365 can become confused if those events are processed at different times leading to appointments that don't show up, appointments that disappear or appointments that can't be modified.
Don't delete
Be sure to never delete meeting requests from your inbox, even if a meeting has been responded to. If you're the person who is responsible for creating appointments, be sure to open the request from inside the mailbox (not from the calendar) and respond to the request with the appropriate reply. Outlook will automatically remove the invitation from your mailbox. Others who may have received the request for the same calendar can delete the request after they see that the request has been responded to when they open it and it says Accepted (or Declined) by XXX at YYY.
To change an entire series of meetings, cancel the original meeting and create a new one.
To change one instance, cancel just that meeting and create a new one to replace it. Always put an end date on a recurring meeting.
A "corrupt" meeting will remain that way until you delete it.
If it is a recurring appointment, delete all occurrences and reschedule it.
Scheduling and Responding to Meetings
Set an end date on recurring meetings you organize
The risk of a calendar item becoming corrupt increases every time you make modifications. Setting a limit on the number of occurrences of a meeting ensures that you start fresh every once in a while and reset the risk factor. We recommend ending a recurring meeting no more than 6-months out and simply recreate it when the time's up.
Don't cancel recurring meetings
Canceling recurring meetings will delete all instances of the meeting--past and future--from everyone's calendars resulting in loss of the historical data. A better method would be to change the end date for the meeting to the day after your last planned meeting so that only future occurrences will be deleted.
Send meeting requests to individuals instead of distribution groups or mailing lists
If you address a meeting request to a group address and then individually add or remove an attendee who was already invited as part of the group, there could be a conflict in the system and the meeting could disappear from that attendee's calendar.
Always send updates when you change a meeting you have organized.
Sending updates to your attendees will ensure that your attendees all have the same, accurate information about your meeting on their calendars or know that it is canceled. If you cancel a meeting without sending an update to the attendees, the meeting only gets removed from your calendar and is then orphaned on everyone else's.
Don't drag and drop
Never drag and drop meetings on your calendar when you need to reschedule. This doesn't always give the option to send an update so they won't know the meeting has changed (see above).
Respond to all meeting requests and send a response
Responding will update the organizer's calendar and help in planning.
Don't respond to meeting requests from within your calendar
Though Outlook allows you to right-click on a meeting in your calendar and respond to it, it doesn't always remove the meeting request from your inbox once you do so. If you subsequently delete the meeting request from your inbox, it will remove the meeting from your calendar. You should only respond to meeting requests from your inbox. See the note above about not deleting items for more information.
Don't forward meetings to other attendees
A forwarded meeting will not add the individual to the organizer's original request and the individual that had received the forward will not receive any updates or cancellation notifications. Ask the organizer to add the attendee and send an update to the meeting if others should be invited.
Alternatives
Different email/calendar client in iOS
Many clients who have had issues with calendaring have had success using Outlook for iOS rather than Apple's built-in Mail and Calendar apps.