I also tried adding an infant and it didn't work.
The agent saw my award was expiring in about 45 minutes, and first said you can't add an infant until you've booked the award. I told her I had been able to do it in the past and she reluctantly did. Rechecked online and confirmed with an agent that my hold deadline wasn't extended. This was for TLV-BKK through El Al, so maybe it depends on route / partner award vs. AA award.
Barely got the ticket booked with 5 min to spare, but they wouldn't waive the telephone booking fee for me