If he is acting like a child it's surely because of his traumas who make him change his personnality or one of his alters. There are people who regressed to the state of babie's behavior, due to the severe traumas. One of your alter ego could be activated, and it's possible that's an inner child who speak through you
This woman was litteraly speaking with a baby voice and acted like one. I don't say that it's the same for Taehyung. But it can explain why you find him regressing and acting like a child. He is not in his normal state