Apple

We have recently changed 20 of our iPhone devices from 6s to X.

For most of the users, the change has been seemless. We have no mobile device management software in place and we manually configure the phones to connect to their Exchange account running of Exchange 2013.

One of the users keeps having an issue where the Exchange Calendar keeps disappearing, then suddenly reappearing, then disappear again. Sometimes, in his "Calendars" menu, his calendars will appear 2 times, 3 times, 4 times but no events will be displayed.

I have tried and confirmed the following :
- The TimeZone is set to choose automatically and is the good one.
- I have tried resetting to factory settings the phone (even though it is a brand new installation).
- The user has not used any iTunes restoration following our basic configuration.
- I have unchecked the calendars for his iCloud.
- The user uses an Apple Watch.
- When the events disappear on the phone, they are still there on the watch.
- When they reappear, the Watch will make sound notifications and vibrate as if the reminders were new.
- He is the only user with this issue.
- He is the only Apple Watch User.
- It was working absolutely fine on iPhone 6S (cannot confirm which iOS version was present)
- He initially had more than 10k events in his calendar and was set to sync All Content. I have archived 95% of the events out of his Exchange Account and choose to only sync 30days of content in his account.

If someone could give me a hint, or simply confirm that I am not the only one with this issue, I would greatly appreciate.