css `color-mix` not working with paint api.

/* working */
--squiggly-line-color: red;
/* working */
--squiggly-line-color: oklch(
    from red calc(l * 7 / 8) calc(c * 3 / 4) h
  );
/* not working */
--squiggly-line-color: color-mix(
    in oklab,
    white 75%,
    black
  );

background: paint(squiggly-line);