Bulk-boundary correspondence, a foundational principle underlying the electronic band structure and physical behavior of topological quantum materials, has been rigorously tested in topological systems that involve conserved charge currents. However, it remains unclear whether bulk-boundary correspondence should hold for nonconserved spin currents. We address this unresolved question by using spin-torque ferromagnetic resonance to accurately probe the charge-to-spin conversion efficiency in epitaxial thin films of a canonical topological insulator, Bi