Primed tnt glitching through blocks

What I expected to happen:
The primed TNT would stay within the enclosed/blocked off cannon and blow it up.

What happened:
The primed TNT was blasted out THROUGH the cobblestone and out of the cannon.

How to replicate:
Build cannon with steps shown below, fill the two dispensers which are facing each other with TNT, fill the one other dispenser with TNT minecarts, flip the lever until all the minecarts have been dispensed onto the rails within the device, push the button.

Evidence:
The video attached (poor quality), the same video in better quality: https://www.youtube.com/watch?v=_3YaQI2lQqE