That's odd. This was content that was still on the MCAT when I took it last year. I even remember keeping the formation of objection permanence occuring ~0-2 years of age on my flashcards.
It’s far from perfect in newborns (containers take some time to understand, and in general infants have weak short-term memory). I also wonder how much effort goes into updating MCAT questions with new scientific developments, especially when there is limited clinical significance - an infant who struggles with object permanence likely has serious neurological/cognitive problems across the board.