tnt from a tnt cannon is glitched on slab

Put the summary of the bug you're having here

What I expected to happen was...:
Ignite the tnt next to the slab

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

Steps to Reproduce:
1. set up a simple tnt cannon
2. put a slab at the end of it so the tnt will fire straight
3. ignite the tnt beside the slab and it glitches up on the slab