Strange Activesync issues and Xs Max

Apple iOS
1

We have some “special people” who have the Xs.Max (iOS 12.0.1). We run exchange2010 onprem. One person has mail, but suddenly lost contacts and calendar, whereas another person has over 1000 contacts but whenever you go in and out of contacts app, it seems wants to resynchronize. Both use iOS native mail.

This occurs across both cellular data and internal wifi.

2

Does the mailbox also resync at the same time?

Have a look for any repeated communication faults between the device and exchange, also capture xCode logs from the phone while the issue happens (this should give you more insight into the contact issue).