You haven't seen my basement. 😬🤣
It is speculation for fun, of course, but if you think in terms of basic developmental physiology or neurophysiology, children's brains aren't capable of the same things that adult brains are capable of, because they're still developing. If you think about how complex human brains are, and how long they take to finish "growing up," it's not surprising at all that she would have problems fitting an adult consciousness into a child's brain. (I can't believe I just wrote that... dayum.)
But cramming an adult consciousness in there wouldn't necessarily create an adult brain-- it would create some aspects of one, maybe? But others aspects would have trouble manifesting due to a "lack of computing power," I suppose?
Also that "adult consciousness" wouldn't have an impact on the other hormonal functions of a kid's body at that age, which would also affect her mental/emotional state. Enough to regress a little? Maybe?
Thinking in terms of Natural Imperatives, children have evolved a lot of specific behaviors adults don't engage in (selfishness, lack of empathy, etc.) that they usually "grow out of," but those behaviors serve a vital evolutionary purpose for the "young," in terms of competing against other "young" for resources and surviving to adulthood, where those behaviors become social disadvantages... for the most part. (Some people obviously don't shed those young behaviors.)
It's just idle speculation, anyway. But I like the idea that human physiology could play an actual role in an isekai, and that the form you take on would start to influence your thinking.