In my case I was credited Correctly at first (real time notification), then my transaction posted a day later after the purchase, and everything was still fine, then it was maybe 2-3 after that, then my available credit suddenly dropped to 2times the amount of the plane ticket (thats x 2).
And it wasn't 72hrs but an entire week went by before my available credit went back to what it was supposed to be.

So it appeared like this:
1. Purchased 25dollar ticket, immediatly online available credit was reduced 25 and showed 75 available/remaining.
2. Few days pass, transaction is posted still showing 75available.
3. 2-3more days pass then available credit suddenly shows 50dollar available even though no other purchases have been made with the AMEX.
4. Week goes by, then available credit is fixed to correct level.

again it wasn't really 25dollar ticket or 100dollar credit line, the ticket was a significant price.

Whatever the case, as long as I dont pay over the limit fees or get stuck somewhere lol.