Hostile Mobs Don't Burn at Height Limit

I was experimenting with changing the 'scaleY' or 'Height Scale' value in customised worlds. Using presets, I set it to -1. What I got was a completely flat land, because it was at the height limit and it cut off everything. However, although it was daytime, loads of hostile mobs spawned and the skeletons and zombies didn't burn.

Also, the trees look really weird.