Comparators connected to brewing stands do not update, when items have been received from droppers. Giving the comparator an adjacent red stone signal or block update, updates the comparator giving the correct output.

All other containers (hoppers, dispensers, droppers, furnaces) update the comparator when items enter them via a dropper.

Advantages:
This allows detection the exact moment brewing is finished with no secondary comparator input.

Annoyances:
Is not consistent with the way in which other containers operate with comparators.