Consider the following scenario:
A user creates an appointment from their Exchange ActiveSync (EAS) client.
The appointment synchronizes into the user's calendar and is visible in Outlook.
The user adds one or more attendees to the appointment from their EAS client.
In this scenario, the appointment in the Outlook calendar does not update to a meeting.
When you add attendees to an appointment, the meeting status of the existing appointment is changed from a value of 0 (the event is an appointment that has no attendees) to a value of 1 (the event is a meeting, and the user is the meeting organizer). This value is removed unexpectedly by Exchange Server.
To fix this issue, install Cumulative Update 11 for Exchange Server 2013.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
Learn about the terminology that Microsoft uses to describe software updates.