Whenever I try to create a Superflat world, if I use any biome-specific Chunk Spawner (even one matching the Superflat world's preset biome) a lake of lava pops up in the lowest 10 or so layers, and nothing else. The chunk's biome still changes, so with other mods, it can be terraformed away in creative and function as intended (including spawning the correct animals), but I even tried it by itself, and I cannot guess what causes this behavior.
Would it be possible to make it function similarly to the default world, so creative builds can experience selective biomes without quite as much terraforming? Or is this too niche of an ask?

Ah, I hadn't seen that before. The behaviour for biome spawners is they take a copy of the chunk generator for the dimension, restrict down the biome list and then use that to generate the chunks - I guess something is going wrong in the process for the superflat chunk generator. I'll have to have a look at what is going wrong.

I'm actually a little tempted to extract the biome changing element of Chunk By Chunk as a separate mod, though I haven't decided how it would work.

Your response is much appreciated- and the idea of the biome changing element being by itself could be a helpful tool for people who make larger builds (like I'm planning on getting back into), so it could be worth messing with! Either way, I hope it isn't too tricky to figure out what's causing the wonkiness, I wish you luck in your coding ventures!!