Same problem with film cameras
with focal plane shutters.
Focal plane shutters are in 2 parts. At faster shutter speeds, as the first half moves, opening the shutter, the second half starts closing right behind it so that a slit of light moves across the film. The shutter is never fully opened.
So we get the same result because the prop is spinning so fast, it's position changes as the shutter moves across the film.
All kinds of weird things happened with those cameras at fast shutter speeds. Bent dragonfly wings, you name it.