In order to determine whether an AppointmentCompose item that has a Recurrence object attached is actually the Series or an Instance we check the seriesId on the item. According to the documentation here this should return null for the series item and the parent id for any Instance of a Series.
This works well for Outlook on Windows and in the Web (as long as the RequirementsSet 1.7 is supported). However on Outlook for Mac this also returns a falsy result for instances. We consider this a bug in the Mac implementation. Can you please check and provide an ETA for a fix?
Looking at the objects we also realize that an the internal state seems to miss several properties on the AppointmentCompose->_data->_data object and has a wrong value of false on the AppointmentCompose->_recurrence$p$2->_isInstance$p$0.