tnt glitching on slabs out of dispensers

Put the summary of the bug you're having here

What I expected to happen was...:
tnt would come out of the dispenser and in the water

What actually happened was...:
it glitched to the top of the slab

Steps to Reproduce:
1. put water right next to a dispenser
2. fill with tnt
3. put slab on top
4. make a redstone circuit hooked to the dispencer