Infant tix has to be in a different reservation for international award tix?

I know there are already several threads related to the infant ticket for the international award tickets, just think this one is a little bit different. So I put it in a thread. Please feel free to merge it.

Currently united.com allows users to book international award tickets with infant. I successfully booked one before online (EWR-NRT-PEK). But this time, I tried to book a new one(EWR-ORD-PEK). The website allows me to finish the transaction, my credit card was charged, but didn't receive the confirmation within 2 hours. So I called the customer service, was told that the infant ticket had to be issued in a different reservation. So she cancelled my reservation and booked the adults' ticket first, and added an infant separately, which has a different confirmation number.

The adults' ticketed were confirmed momentarily, but after 12 hours, I haven't seen the confirmation for the infant one, and the infant hasnt showed up under my revervation, as was discussed several times here...

I was wondering why the infant ticket can't be booked within the same reservation of the adults as my previous experience. Is it always the case or I was misinformed?

Thanks in advance!