<p>Filtered tailings stacking has emerged as an alternative to conventional tailings dams, particularly under increasingly variable climatic conditions. However, its performance is strongly influenced by moisture dynamics in the unsaturated zone, which will demand an improvement of technical knowledge. Existing studies often neglect the combined effects of compaction variability and hydraulic parameter uncertainty, which are important for reliable modeling internal suction and moisture behavior. This study aims to investigate the influence of void ratio and hydraulic parameter uncertainty on internal moisture distribution and suction development in filtered tailings stacks. Soil–water retention curves were calibrated for three compaction states using Bayesian inference, and the results were propagated through a series of Hydrus-2D simulations representing a two-stage stack under atmospheric boundary conditions. All simulations assumed identical initial moisture content and hydraulic conductivity to isolate the role of void ratio. Results demonstrate that the variation in void ratio influenced internal suction up to four times more than hydraulic parameter uncertainty, acting as the dominant factor in the system’s response. It was found that more compacted tailings develop higher suctions which, although delaying surface saturation, create gradients that promote deeper moisture redistribution over time. These findings highlight the importance of accounting for in-situ variability in void ratio during design and performance assessment of filtered tailings stacks.</p>