Minecraft Height Limit Showing Too Early

What I expected to happen was...:
I can build extremely high in the sky up to hundreds of blocks.

What actually happened was...:
The height limit showed way too early. It showed at 18 blocks and still said 256.

Steps to Reproduce:
1. Go into Edit World.
2. ... Build up 18 blocks in the snow biome.
3. ... Height limit message shows on the 19th block.