The discovery of higher-order topological phases hosting boundary modes at hinges or corners has revolutionized our understanding of quantum materials. While axion insulators (AXIs) characterized by symmetry indicator ${\mathbb{Z}}_4=2$ are known to host chiral hinge states, the topological landscape of magnetic crystals with a trivial indicator ${\mathbb{Z}}_4=0$ remains largely unexplored. In this work, we demonstrate that a trivial symmetry indicator can mask a rich variety of topological phases, including Weyl semimetals with even pairs of Weyl points, trivial insulators, and weak second-order topological insulators (WSOTIs). We find that these WSOTIs are rooted in fragile topology and support robust chiral hinge states that mimic the signatures of AXIs, despite their distinct topological invariants and contrasting spin-polarization patterns. Through Wilson loops analysis and symmetry-compatible orbital coupling, we establish a generalized bulk-hinge correspondence for WSOTIs and demonstrate that external magnetic fields and surface engineering can precisely control the propagation of their hinge states. Finally, we propose a WSOTI-AXI heterostructure that functions as a tunable topological spin splitter. Our results provide a general framework for uncovering higher-order topology in fragile phases and suggest a platform for topological transport of spin polarization.

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• Berry curvature
• Chern number
• Geometric & topological phases
• Hinge states
• Topological insulators
• Topological phases of matter
• Topological materials