The DOT 24 hour hold rules no longer apply, since AA switched to offering 24 hour refunds instead. So the only guarantee is what AA promises, but that's always been until 11:59 the next day.
I've ticketed held fares beyond 24 hours, fares that are from sales no longer being offered, fares from buckets that had since sold out, and even fares where the minimum purchase deadline had passed. 10 or 15 years ago, occasionally a held reservation would show a price increase, but a call to web services would always restore the original price; but I haven't seen that in a long time, and even then, the reservation wasn't canceled.
Could this have been a time zone issue? Looks like OP posted close to midnight, so perhaps after midnight in the time zone the reservation was held in. That's usually (always?) the time zone of the initial departure city. The email from the hold should clearly show the time zone for the deadline.