Parts of Super-Flat World Not Flat

I Generated a super-flat world and in the distance i saw trees so i flew in their direction and chunks were generating like normal chunks.

What I expected to happen was...:
My Super-Flat world would stay flat.

What actually happened was
A large area of chunks were Generates as if they were normal Non Super-Flat Chunks

Steps to Reproduce:
1. I Generated my super-Flat world on the Over World Preset.
2. I enabled cheats structures and kept bonus chests off
3. I don't think it matters but i named the world Tank Testing World. (i was about to experiment with build tanks in game)
4. I named 3 Villager spawn eggs Frank, Bert and Ernie.1 Blaze was named Sir. Williamsferguson and 1 Ghast was named That One Guy.
5. Chunks started to spawn as if they were not superflat