That is crazy indeed.

I can't remember, do the other encoders automatically change the chroma subsampling? XYB JPEG peaks pretty hard at specific bpp ranges depending on the subsampling.

Also, things like this make me wonder what other tasks would be better in XYB colorspace. BM3D, for instance, performs best in "opponent" color space: https://github.com/HomeOfVapourSynthEvolution/VapourSynth-BM...