Improving constraints on the basal ice/bed properties is essential for accurate prediction of ice-sheet grounding-line positions and stability. Furthermore, the history of grounding-line positions since the Last Glacial Maximum has proven challenging to understand due to uncertainties in bed conditions. Here we use a 3D full-Stokes ice-sheet model to investigate the effect of differing ocean bed properties on ice-sheet advance and retreat over a glacial cycle. We do this for the Ekström Ice Shelf catchment, East Antarctica. We find that predicted ice volumes differ by >50%, resulting in two entirely different catchment geometries triggered exclusively by variable ocean bed properties. Grounding-line positions between simulations differ by >100% (49 km), show significant hysteresis, and migrate non-steadily with long quiescent phases disrupted by leaps of rapid migration. These results highlight that constraints for both bathymetry and substrate geologic properties are urgently needed for predicting ice-sheet evolution and sea-level change.