Symptoms
After you install Cumulative Update 14 for Microsoft Exchange Server 2013 and Cumulative Update 3 for Exchange Server 2016, public folder posts may not be indexed if a public folder migration is active. Public folder search results may be missing some items.
Resolution
For Exchange Server 2013
To fix this issue, install Cumulative Update 15 for Exchange Server 2013 or a later cumulative update for Exchange Server 2013.
For Exchange Server 2016
To fix this issue, install Cumulative Update 4 for Exchange Server 2016 or a later cumulative update for Exchange Server 2016.
Note After you install this update on either version of Exchange Server, you should move the public folder mailbox to a new database to make sure that all items are indexed correctly.
Status
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
References
Learn about the terminology that Microsoft uses to describe software updates.