When a user accepts a meeting request that's just a single occurrence of a recurring meeting by using the Microsoft Exchange ActiveSync client protocol (the version is earlier than 16.0), the user is added to the entire series of this meeting as an optional attendee.
This issue occurs because no recurrence information is passed to the Exchange ActiveSync client protocol.
To fix this issue, install one of the following cumulative updates:
Cumulative Update 9 for Exchange Server 2016 or a later cumulative update for Exchange Server 2016
Cumulative Update 20 for Exchange Server 2013 or a later cumulative update 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.