I updated to 0.2.44 from 0.2.40b, and then decided I wanted a creative testing world. However, when I tried to customize a superflat, exiting the superflat customization screen dropped me in the create world screen with the world type set back to "Default". This also occurs when attempting to choose which biome a Single Biome world should contain. I don't know if this was a problem before 0.2.44 because I haven't tried to make a superflat or single biome world before in this pack.

If you return to selecting Superflat after attempting to customize it and the world type being set to Default, it uses the first/default option for a superflat, the 3-4 layer world that gets you all the slimes everywhere. Haven't seen what it does for Single Biome.

I am also having this problem in 0.2.45. I think this is a problem with TerraForged. I made an instance with TerraForged only without other mods and it is also doing this.

This appears in logs when you press "Done" on the superflat preset editor:

[Render thread/INFO]: Re-seeded TerraForged generator: -4695520290737441496
[Render thread/INFO]: Attached generator profiler