Strain localisation, in which deformation concentrates into narrow shear bands, is central to many geotechnical failure processes but remains difficult to model reliably. In classical strain-softening continua, localisation is associated with loss of well-posedness, causing pathological mesh dependence: the predicted band thickness collapses towards the resolution of the discretisation and post-peak responses become physically meaningless. By incorporating the influence of grain-scale rotations and dimensions, micropolar, or Cosserat, continuum theory has long been recognised as a rigorous and physically meaningful way to address this problem in granular media. Motivated by large-deformation failure scenarios, this contribution presents a finite-strain micropolar material point method (MPM) framework for simulating localised deformation in a robust computational setting. The focus is the elastoplastic form of the method and its consequences for modelling localised failure in geomaterials. The formulation introduces independent microrotations, couple stresses and internal length scales, allowing shear-band thickness to be controlled by material parameters rather than by the background grid. After a brief outline of the relevant micropolar kinematics and constitutive structure, the method is shown to remove the pathological mesh dependence observed in an equivalent classical formulation. The main results demonstrate mesh-objective shear bands of finite thickness in frictional, strain-softening materials. Large-deformation examples, including biaxial compression and column collapse, further reveal that localisation need not remain a fixed planar feature: shear bands may progressively curve, reorient and evolve as deformation proceeds. These results suggest that micropolar theory provides more than a numerical regularisation: it offers a mechanically grounded framework for investigating how localised failure initiates, develops and interacts with its surroundings in geomaterials.