The most likely cause is changing reflections, not strobes or a special shutter effect.

A spinning prop normally blurs smoothly. But if one side of the prop has a shiny metallic leading edge, it can reflect light much more strongly at certain angles. In this case, the lower blade arc is likely the side whose leading edge is facing the camera, so as it rotates it catches and loses reflected light repeatedly. That can make one continuous blur look like many separate blade positions.

The reflected light may be coming from the low sun and/or bright reflections off nearby water, which can create multiple bright highlights across the prop’s path. The upper arc would not show the same effect because its blade orientation is different, so it doesn’t direct those reflections toward the camera.

A secondary possibility mentioned is slight torsional vibration of the propeller, which could subtly change the blade’s angle and therefore its reflections, reinforcing the segmented look. But the key explanation is angle-dependent specular reflection from the prop’s shiny leading edge.