I created a superflat world that was supposed to be a desert with a single layer of bedrock and 4 smooth quartz layers; nothing else.

When I spawned in, the world was filled with lava lakes (which I was sure required its respective tag in the superflat generator). I increased the quartz layer to y=63 and found that both lava lakes and amethyst geodes generated in a supposed to be blank superflat world.

Superflat Customizer: "minecraft:bedrock,63*minecraft:smooth_quartz;minecraft:desert"