Help making underground structures/features in 2023.4 with a fabric loader

So I'm struggling to make features spawn underground with version 2023.4, with a fabric loader for a modpack with friends. There's no "swapping to a different version" or "just don't use fabric."

Structure or feature, nothing spawns underground. I've tried letting the features spawn in without restrictions, but nothing spawns below the surface. This happens in all dimensions; overworld, nether, even custom ones! (haven't tried in the end because i have no reason to) I feel like I've tried everything, but am blocked by unknown and unneeded restrictions.

Is this because I'm not understanding how to put something underground, or is it because the fabric loader plugin prevents it from working?