A 4-year-old boy was removed from a severely neglectful home environment at age 2 and placed with a stable foster family. Despite 2 years in a nurturing environment, he rarely seeks comfort from his foster parents when distressed, shows minimal social and emotional responsiveness, and displays limited positive affect during interactions. He does not approach unfamiliar adults and instead appears emotionally withdrawn and inhibited. His foster parents report that he sometimes exhibits unexplained episodes of irritability and sadness without identifiable triggers. He does not have intellectual disability, and autism spectrum disorder has been ruled out through comprehensive developmental assessment. Which feature most clearly distinguishes this presentation from disinhibited social engagement disorder (DSED)?