A public health researcher in Mumbai conducts a case–control study on the association between oral contraceptive use and myocardial infarction (MI) in women aged 30–50 years. Cases are recruited from the cardiology ward of a tertiary hospital, and controls are selected from the outpatient department of the same hospital. The study finds a strong positive association between oral contraceptive use and MI (OR = 3.2, 95% CI 2.1–4.8). However, when the researchers stratify by smoking status, the association becomes much weaker in both smokers and non-smokers. What type of bias or confounding is most likely operating in this study?