some iOS EAS clients displaying deleted mail

Hi,

I've got an on prem Exchange 2013 CU6 deployment.  Some of my users with iOS EAS mail clients see deleted mail remaining in their phones' inboxes.  This is also true with mail that's been moved from the inbox to another folder.  It's an intermittent problem.  

Removing & re-adding the phone will temporarily fix the problem.

Any thoughts or pointers?

Thanks!

Adam