I had several theories, hoping that one of them was the case. It wasn’t uncommon for anyone who went through immense pain to sequester their conscience deep into their mind. At that point, the body would mostly run on auto, going through the motions. It’s possible that when Bellatrix and her posse tortured these two to the point where their more, recent memories were pushed far back into that small space. Leaving only their childlike self behind. 