Do you mean the Sync Issues folder returned by the command Get-MailboxFolderStatistics?
Sync Issues folder is a container for all synchronization logs. The data in the Sync Issues folder is local and not copied to the server.
Details are introduced here: https://support.intermedia.com/app/articles/detail/a_id/12123/type/KB
And you could refer to the related thread here discussed the issue: Purge/delete all SyncIssues in Exchange mailboxes
The contents of the Sync Issues folder are not copied to your server, and you cannot view the items in the Sync Issues folder from any other computers. So sync issue folder didn’t occupy the space of mailbox database."
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