the "customize" button, when making a new world of type "Custom Cubic", no longer works in any version above 1.12.2-0.0.39.1. without this feature, it is impossible for the world to generate landscapes above or below the vanilla build ceiling/floor, like with version 1.12.2-0.0.39.1

issue posted for the wrong mod. Issue reposted on "cubic world gen" page