This is an interesting data point:
Purchased 4 X $50 giftcards Sept 8 (posted as a single $200 transaction) - no credit applied so far
Purchased 1 ticket with AS Companion Voucher from AS Visa Sept 12.
Sept 18 - received Amex Credit for the Tax/Fees on the Companion voucher - which was the only amount separately listed on the Amex transactions (although my AS receipt shows the amount as exactly $1 more!)