Why Mach bands are not seen at the edges of a step.

Unlabelled: Mach bands are seen at the two ends of a ramp in luminance from one uniform level to another. Narrow, sharp-edged stimuli centered on the ramp attenuate both Mach bands simultaneously.

Conclusion: Mach bands are not seen at the edges of an abrupt step change in luminance because the sharp edges actively suppress them.