A 52-year-old man with a history of hypertension presents with progressive weakness, proximal muscle pain, and hypercortisolism. Laboratory investigations reveal: 24-hour urinary free cortisol 450 µg/day (normal <100), morning plasma cortisol 32 µg/dL (normal 5–25), and ACTH 8 pg/mL (normal 10–50). A high-dose dexamethasone suppression test (8 mg overnight) shows cortisol suppression to 8 µg/dL. Which feature best distinguishes ACTH-independent Cushing syndrome from ACTH-dependent Cushing syndrome?