If you're using batch migration or serial migration to migrate public folders from Exchange Server 2007 or 2010 to Exchange Server 2013, 2016, or Exchange Online, data loss may occur if all of your folders' replicas aren't on the primary database (the primary database server which the migration service connects to). Data which is in the folders that are being migrated is copied, but any incremental changes that are posted after the initial sync may be lost.
A workaround is available if you are unable to install the updates which fix this issue: Before you start batch migration, make sure that all public folders have a replica on the public folder database that will be specified as the source for migration. Also, make sure that the public replication is in sync.
See Data loss may occur during public folder migration to Exchange 2013, Exchange 2016, and Exchange Online for more information.