If you need to stop Outlook from showing messages Inside the Synchronization Log folder known as Sync Issues folder follow the step below:
For more Outlook posts see:
Sent Items And Deleted Items Behavior In Shared Mailbox Exchange Server 2016
Open Outlook -> Right click Unread Mail folder -> Customize This Search Folder
Tick the Inbox Folder and Un-tick the Search Subfolder.
Sync Issues Folder:
Unread Items before change.
Unread Folder after Change