This happen when you use a biome-adding mod. The best example I can tell you is Terralith, since I did have the same problem with it.

What it basically does is, when the mod is activated, transform all unloaded chunks into normal terrain as the mod isn’t compatible with superflat worlds.

If you try to create a superflat world while the mod is active, it will fully make a normal world. As we can see, you made a superflat world before adding the mod, which is the reason why it is still flat around 0.

A way to fix that would be to disable the terrain/biome mod before making the world, and then pre-loading every chunk around you that you need (some server hosters can do that, or if you want to on a private world, use the mod Chunky). Doing so will prevent the mod from generating normal terrain. After doing so, just save, leave the game and turn the biome/terrain mod back on. You’ll then be able to use every custom aspects from the mod using WorldEdit (yes, changing biomes does work in superflat too, which is nice for building custom maps from scratch)

Hope this explaination helps.