A 28-year-old woman is brought to the psychiatry outpatient clinic by her mother. She has been on buprenorphine 16 mg/day for opioid use disorder for 6 months and reports good adherence. However, she now complains of persistent low mood, anhedonia, and fatigue despite stable opioid abstinence. She denies active opioid use on urine drug screening. Her psychiatrist suspects that her depressive symptoms may be related to the buprenorphine itself. Which of the following is the most accurate explanation for opioid-induced depression in this context?