https://gitgud.io/LynxChan/LynxChan/blob/master/src/be/engine/uploadHandler.js#L471
Yep, 4x4 sampling trips up Imagemagick.
But muh low score of 1634 bytes.
Have this grayscale, probably absolute minimum possible to post (bar the all gray image, believe me been there done that) jpeg as a substitute.