Delete the account fully from iOS and re-add it. (Do not just "uncheck" the Contacts option in the Accounts area in iOS.) Also, you MUST delete the synced device in the connectivity area in webmail prior to re-adding it to your iOS device. This will be a 100% removal of all synced info in iOS and SmarterMail and will give you the best results for solving the problem.
There have been 3 different reports of this issue so I decided to sit down with one of my developers and we tested FIVE different iOS devices, using the same account connected to a customer's server in their production environment, and we had ZERO issues.
Unfortunately, we are seeing a TON of mail issues from Apple across iOS and MacOS (SmarterMail, Exchange, and all protocols) which is making things very difficult. If the above suggestion did not work, issues are often resolved by backing up your iPhone or iPad to iCloud and then Factory Reset your device and then restore from iCloud. iOS is starting to have bad upgrade issues and FRESH installs / restores have much more consistent results. This wasn't the case for Apple and was something we were used to seeing only with Microsoft Windows. I hate giving this suggestion because it's almost as bad as "try turning it off and on again" :). But it really is starting to become a thing for iOS.
We did just sit down with the Apple Business Team on a variety of issues this week and expressed some of our concerns. They joked about how Apple uses IMAP and DAV for all the corporate mail and that their native Mail applications (iOS and MacOS) don't get used internally with EAS / EWS. This explains why there are so many mail issues across all their operating systems.
In our release notes, we also mentioned that Apple has BETA's of iOS, iPadOS and MacOS. There are NO new features and these upcoming releases are just FIXES. There are many mail issues resolved in the updates and we look forward to testing them to see what they resolve.
I'm not eliminating us as an issue, I know we can always be a culprit or in some cases find a way to work around a third party's problem but this particular issue we have looked at numerous times and these are the results.
Hope this helps and I look forward to your results. If you are still having an issue, don't hesitate to PM me and we can test again on your production account as well.