Sidenote regarding infants in general:
I've been advised by customer service, that the amount of infant oxygen masks on each aircraft is limited. That means, if you're going to book an infant to fly with you, do so from the very beginning, as it is possible, that if there are enough infants already booked, you won't be allowed to fly with your infant at all! (or at least that what she told me)
Another issue I came across myself lately is this: I knew you can't book the exit rows when flying with an infant. So I booked the row behind the exit rows. I then had a phone agent add an infant to that booking and she (the same who advised me about infant masks) assured me that everything was fine. By pure luck I did a half dummy-booking a few weeks later and noticed, to my horror, that my seat appeared as available for anyone to reserve! I checked my reservation again, it showed the same seat I was supposed to be booked in just fine! Called again, new agent, she first guessed that it was an issue that had to do with the seat being a "B" (middle seat) and said something about baby masks only being available in window seats, but then she noticed that the system would allow a re-location to a middle seat in a different row, so she came to the conclusion, that it's not only the exit row seats that need to be kept "infant-free", but also the +1 and -1 rows to those exit rows.
Anyway, ended up way in the back now, so my ticket also got kind of messed up, but ...whatever
. At least the reservation didn't vanish into thin air like the OP's did.