Thanks for the info.
MPC-HC comes bundled with LAV filters.

I hadn't tested this as I have no interest in the release itself to download it, however the uploader has said on nyaa it works fine on mpc-hc (not sure which renderer).

Thanks for raising the issue.
Assuming 'vs' = VapourSynth, could you share what input plugin you're using and if there's any parameters you're passing in? I tried L-SMASH and BestSource but both failed (to my knowledge, both rely on libavcodec under the hood, which FFMS2 also uses).

The video also seems to be completely garbled in MPV and VLC. The LAV DirectShow filter seems to decode it fine though.

ffmpeg doesn't like it and prints errors.
Given this, I'm inclined to see this as a corrupt video (which just happens to work in the LAV DS filter).

444 still has its place when you don't want to downscale the chroma further, see motbob's bleach DVD encode. And the rendering issue can be fixed by letting vs do the screenshots instead.

ffmpeg can't really do Hi444 screenshots. You get the same exact thing if you were to put those files on a Plex server and use the auto generated thumbnails.

Hi444 encodes are a remnant of the past, when encoders for most fansubbing groups didn't really know what they were doing at all.