Multiple Reddit members report that Audible runs the monthly subscription charge a few minutes before midnight of when the next month begins. Thus, the monthly charge for a membership that renews of the first of each month will actually charge a few minutes before the beginning of the next month. (Apparently the digital credit goes with the swipe/charge time, not the posting time.) If you already maxed out your prior month, you won’t get any AmEx credit to offset the charge.
If they do this consistently it probably won’t matter since you’ll always have your Audible paid up with the prior month’s credit. But it’s probably worth just avoiding having a 1st-of-month renewal date for Audible. Or for any of the other subscriptions too, for that matter.
Reports are that, upon request, Audible is amenable to crediting back the charge and recharging you again. That should work to get the missing credit.
(Here’s our Amazon/Audible affiliate link for those signing up to a new Audible subscription.)