When you place a block at build height and then break it, the area below will not cause zombies to burn.
To reproduce, start a new world, build a tower to build height (which is now 256 and you will get a message when you get there) of any solid block like dirt.
Now build a small platform 3 or more blocks below it.

Now shade halfof this platform

Now break the shading blocks and place zombies down on both sides of the platform. Notice how they only burn on the one side that never had the blocks over it.