Exchange email randomly deletes from phone

Dealing with a user who has multiple email accounts configured on her iPhone 5, one of which is an MS Exchange account (Office 365). Occasionally, the entire contents of the mailbox will delete from the phone and subsequently re-synchronize which can take days depending on the network speed. We have deleted and reloaded the email profile, checked various settings, spoken to Microsoft who verified the account is configured properly, etc.

We thought it may have been an issue with the device but the same problem was happening on her old iPhone 4S, which prompted her to purchase the 5. Same issue exists and we have no idea what is causing it.