At launch, it was possible to create and use custom heightmaps with your custom planets. Ever since at least 8 months ago, around patch 1.3, trying to use these custom heightmaps results in blank planets with no heightmap at all. This isn't a problem of file settings on save, either, since if you copy an existing heightmap and rename it to the same thing as what's in your [Mod]_ContinentDefs file, the game still won't use it. It's as if the game only allows you to use a default heightmap or randomly generate a new heightmap on custom planets every time you play the game.
Why has this been broken for so long?