bug with tnt and slab under water

I wanted to kill a chicken with tnt in the pool water that I had made, but when the tnt exploded It broke two slab, quartz and netherbrick.
1: Make a hole 3x3x3
2: Line the bottom half slab
3:Putting water into the hole
4: Ask a tnt middle and explode
5: Normally at least half slab is exploded