Question

Are Monthly Active Rows (MAR) counted for historical data in priority-first sync? 

Answer

Fivetran will count MAR for all the syncs running on the priority sync feature except the very first sync, including the ones that are fetching the backdated/historical data. Please see the detailed documentation here. No action is needed at your end.

Cause

Priority-first sync counts MAR for all the sync except the first one.