We are having an issue where multiple users are missing calendar items prior to 2016. All meetings prior to the time where it cuts off are not visible within an Outlook client or OWA. None of the users in question have auto archiving turned on. We do have an archiving solution in house, but we do not archive calendar items and all of the users in question have different dates to when it stops showing meetings in 2016. Looking at other calendars, specifically shared calendars, these all go back to the initial setup of Exchange 2010 or when the shared calendar was created. Exchange is on premise and everything is installed on the same servers/VM's since deployment.
Is it possible that the use of an iOS or Android device may have caused the meetings to disappear if there was a sync issue? Around the time the meetings disappeared, we had a huge push for users to stop using the native email programs on their devices and move to the Outlook app.