We should be able to Track Inbound Appointments from Outlook when ACT Status is Outlook in Mailbox although Incoming+Outgoing Status is ServerSideSync.

The reason is that the customers want to use Folder Level Tracking with external Appointment tracking.
Folder Level Tracking requires Server Side Sync in Mailbox and external Appointment tracking requires Outlook in Mailbox.
If we keep these options in the Mailbox, when we track such incoming appointments we get the error= "You can't track an appointment that was created by another user" .
The reason for the error is that the IncomingEmailDeliveryMethod in the code looks for Server Side Sync rather than Outlook when we track inbound appointments and have below configurations in the Mailbox:
1)Incoming+Outgoing+ACT status = Server Side Sync.
2)Incoming+Outgoing status = Server Side Sync ; ACT status = Outlook.

Also find similar issue here: https://community.dynamics.com/crm/f/117/t/176775