Bell or Clauser-Horne-Shimony-Holt (CHSH) tests on superconducting quantum processors are commonly interpreted under the assumption that repeated circuit executions sample a single, stationary-preparation ensemble. Here, we show that this assumption can be violated on contemporary hardware, with direct implications for the interpretation of observed Bell violations. We introduce an ensemble-divergence framework in which the slow temporal drift of the preparation process induces context-dependent effective ensembles, even when measurement independence and locality are preserved. This leads to a relaxed Bell bound $\left|S\right|\le 2+6{\delta }_{\mathrm{ens}}$, where ${\delta }_{\mathrm{ens}}$ quantifies preparation nonstationarity. Because ${\delta }_{\mathrm{ens}}$ is not directly observable, we develop an operational witness ${\delta }_{\mathrm{op}}$ based on bin-resolved outcome statistics for fixed measurement channels. Using Pauli-axis measurements on IBM superconducting processors, we observe statistically significant operational drift that persists after full two-qubit readout mitigation, ruling out measurement artifacts. In contrast, drift extracted from CHSH-optimal measurements is eliminated by mitigation, demonstrating that such settings are unsuitable for diagnosing preparation nonstationarity. We further show that the observed Bell violations imply only modest ensemble divergences, comparable in scale to those required in Hall-type measurement-dependence models, but arising here solely from preparation drift combined with experimental scheduling. While balanced or shuffled scheduling mitigates the operational manifestation of this effect, it does not remove the logical role of the stationary-preparation assumption in the CHSH derivation. Our results identify preparation nonstationarity as a distinct assumption that must be assessed when interpreting Bell-test statistics on noisy intermediate-scale superconducting devices, and highlight the need for drift-aware protocols for reliable quantum certification.

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• Open quantum systems & decoherence
• Quantum computation
• Quantum correlations in quantum information
• Quantum correlations, foundations & formalism
• Quantum measurements