Having memories of a past life and abruptly changing into that person upon getting them seem like two different things to be honest. We're sort of used to the "I remembered my past life, I'm that person again now" thing from various isekai series by now, but just getting the memories on their own seems like it could be more along the lines of having watched a really long movie or something. When you were a kid, things you saw weren't always understood immediately either. Ever watch an old movie or such from your childhood and suddenly realize "Oh wow, I did not catch that before" about something in it? Maybe the setting of the story was way darker than you thought or there were things left in there specifically for adults to pick up on like a sneaky dirty joke.
Seeing things and having experienced them or understanding them could be fairly separate matters here. She could remain the little girl who was just used to being locked in a box, because that was her normal. The past life memories might be the thing that seems weird to her if regaining them didn't immediately attach a her to her former sense of self as well. Loving parents that don't lock their kid in a box while the fuck strangers for money? "How peculiar." I don't know, man. Either way around is still about something it's not like any of us have ever experienced. When's the last time you woke up with your past life's memories? I sure haven't done so thus far.