This is much more likely to be a shutter problem than an image processor issue. The reported symptom matches a dragging or uneven first shutter curtain: part of the frame is exposed for slightly less time, creating a bright/dark band.

Because it happens in different lighting and exposure settings, and changing shutter speed hasn’t solved it, the mechanical shutter is the main suspect. In practical terms, one shutter curtain may not be moving consistently across the sensor.

The usual remedy is camera service/repair. As a quick check, try comparing mechanical shutter vs. any electronic shutter mode your camera offers; if the banding only appears with the mechanical shutter, that further points to the shutter mechanism.