Fossils and amethyst geodes generate floating in customized worlds with no terrain

As shown in the attached screenshot, in a customized world with no terrain, the fossil and amethyst geode features generate floating. This can be reproduced by generating a world using the attached settings file (which is the same as the one in MC-201704) and going to (-386, 633). Fossils have been affected since at least 1.16.3.

Expected Behavior:

Like Mineshafts, Strongholds, and dungeons, these normally underground structures should not generate if there is no ground to generate under.