Mobs will spawn in at y=256 when the light level is above 9 both on server and client

Put the summary of the bug you're having here

What I expected to happen was...:
No mobs spawning at any y-variable above light level 9.

What actually happened was...:
Mobs Spawn in at y=256 above light level 9.

Steps to Reproduce:
1. Build up to y=256 using only whole blocks such as dirt, glass, etc. and NOT slabs, carpet, etc.
2. Wait until both client light and server light are above 9.
3. Mobs spawn in.