[Jan Wassenberg]

Look like chroma subsampling is indeed the culprit here.
It is a viable technique on smoothly varying images because the eye is not as sensitive to chroma. However, here there is a high-contrast edge, so the artifacts are more likely to be perceived. Presumably ProRes is not going to any great effort to cleanly separate luminance and chrominance, so it’s even more visible.

Using 444 is a good workaround.