We systematically investigate the stability of monolithic perovskite/silicon tandem solar cells under reverse voltage bias. Specifically, perovskite-limited, current-matched, and silicon-limited conditions are analyzed using equivalent circuit modeling and controlled experiments to clarify the subcell voltage distribution and failure behavior under reverse bias. We find that silicon-limited tandems show superior stability, retaining over 95% of their initial efficiency under prolonged reverse-bias stress, as most of the applied voltage drops across the robust silicon subcell while the perovskite remains in forward bias. This protective effect is strongly governed by the silicon shunt resistance: a high value maintains protection of the perovskite top cell, whereas a shunted silicon cell results in voltage redistribution and tandem failure. High air mass solar spectra in the early morning and late afternoon, characterized by diminished blue irradiance, can transiently shift the current-limiting subcell from the silicon to the perovskite cell, leading to increased susceptibility to reverse-bias stress. Fine-tuning the top-cell bandgap based on these diurnal spectral conditions can enable silicon-limited operation to be maintained throughout the day, offering a practical route to enhance tandem resilience without structural modification. These insights provide spectrum-aware, practically actionable guidelines for designing reverse-bias-resilient tandem solar cells.