Create Gradient Mesh's To Center / To Edge highlight lightened every point
towards RGB white, so a CMYK (or grey) point became display RGB. A patch with
any non-CMYK corner then mixes in RGB, and the PDF writer separated those
colours back to CMYK with K = 0, moving ink off the K plate. Points now
lighten towards white in their own colour model (no ink for CMYK), so the
mesh stays CMYK and K shrinks with the other inks. Flat meshes (highlight 0)
already kept their inks.