A 42-year-old man with a 10-year history of poorly controlled type 2 diabetes mellitus presents with a 1-week history of severe fatigue, weight loss, and recurrent infections. His fasting blood glucose is 320 mg/dL, and serum β-hydroxybutyrate is elevated at 4.2 mmol/L (normal <0.6). However, his arterial pH is 7.32 (mild acidosis), and HCO₃⁻ is 18 mEq/L. He denies polyuria, polydipsia, or altered mental status. What is the most likely explanation for the elevated ketone bodies WITHOUT severe metabolic acidosis in this patient?