A widespread experimental realization of quantized anomalous transport in the intrinsic magnetic topological insulator ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ is hindered by its elusive surface magnetic gap. Uncovering the origin of the gapless states is thus essential for accessing its topological properties. Here, we show that surface defects lower the electrostatic potential, drive topological surface states into subsurface layers, and suppress exchange interactions, thereby closing the gap. Tuning the surface electrostatic potential via external electric fields or interfacial fields in van der Waals heterostructures restores the expected gap and enables control of its topology. This is confirmed by model calculations and validated in defective ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ films interfaced with polar insulators, explaining the enhanced quantum anomalous Hall effect under ${\mathrm{AlO}}_x$ capping observed in recent experiments. Our theory identifies electrostatically driven surface-state delocalization as a competitive origin of gap suppression and proposes displacement-field engineering for robust quantized transport.

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• Electronic structure
• Topological phases of matter
• Magnetic systems
• Topological materials
• First-principles calculations