Physiologically, this makes sense. If an adult's "consciousness" were crammed into the brain of a much younger child, the child's physiology would naturally have a great impact on that adult "person's" perception of the world, their emotional/mental health, etc.
It's kind of like trying to jam a 6-pack of beer into a cooler bag that will only hold 4 cans. It doesn't quite fit well, and the container isn't quite suited for it, so you're going to have to get rid of a couple of cans, or else the cooler bag is going to break. In this case, the cooler bag is her new brain, so breaking it might be a bad idea... so she regresses a bit so her consciousness can fit in the new brain more easily... or something like that, maybe?
It's always hard trying to figure out if these isekais are going to play it seriously or if they're going to fake me out and wind up playing it dumb after all.