There were certainly times in the past where various Amex credits including this one has taken 10 days or even 2 weeks to credit. Specifically this Saks credit I recall there was once that many of us, including me, waited ~4 weeks.

There are ways that the retailers can report to Amex what the purchase was for and they/you can see it in the details. For example, UA Travel Bank, it says in the transaction details that that's what it is but until recently those transactions were consistently reimbursed anyway for the airline fee credit. This was going on for quite a number of years. Very recently there have started to be some reports that no reimbursement was received so it's possible that Amex is using these transaction details to stop that type of transaction from being reimbursed as its technically not an eligible charge for that benefit.

The same can be true for this Saks credit. Yes, in the past there have been many DPs of receiving the credit for $50 gcs but there is no guarantee that this loophole will continue to work forever.