Invisible flicker, visible bands
Changing LED output and sequential sensor readout can produce bands rather than simple underexposure.
Rows may not record the same instant
Some electronic shutters read sensor rows sequentially. If source brightness changes during that readout, rows can collect different exposures. A mechanical shutter is not a universal cure: light-driver behavior, shutter time and readout determine the result. Video pulsation and still-image bands need not look alike.
Retest after changing the dimming level
Some LED dimming methods alter temporal behavior with output setting. A light that works at full power can cause trouble lower down. Test the intended shutter mode, speed and output across several frames. Test video at its actual frame rate and shutter setting, including any extra fixtures and displays on set.
Different rows, different sampling times
A sinusoidal source is read sequentially over 20 ms. PWM, sensor response and mechanical shutters are omitted.
Fine shutter adjustments can help under specific conditions
Ordinary flicker reduction and high-frequency fine adjustment have different support conditions. Consult the camera manual and inspect real test frames; a setting suited to one source may not suit another. The model below samples a sinusoidal source rather than measuring real PWM, sensor response or shutter travel.