We investigate Pauli spin blockade and the influence of hyperfine interaction for a system of six-electrons in a device composed of two weakly coupled vertical quantum dots. As a consequence of a large built-in offset energy between the dots, the relevant charge configurations consist of one (zero) and five (six) electrons on the upstream dot and downstream dot respectively. On sweeping the out-of-dot-plane magnetic field up and down, abrupt steps and hysteresis in the leakage current are observed in the Pauli spin blockade region that evolve systematically with bias. We propose a model to explain our observations in terms of hyperfine induced singlet-triplet mixing.