"You can’t track an appointment that was created by another user" error when synchronization is turned off

Hi All,

We use CRM 2016 Online and use "Microsoft Dynamics CRM for Outlook" synchronization mode.

Also we set "Appointments, Contacts, and Tasks" to None for global Email Setting and for each user mailbox.

But when user tries to make "Set Regarding" operation on some appointments, he gets "You can’t track an appointment that was created by another user" error instead of "Synchronization tasks can't be performed on this computer because the synchronization method is set to None."

Why could it happen?

Also users can press "Track" on the appointment, but there is no error message appears that it is impossible and appointment does not appear in CRM.