An unexpected glitch when a TNT minecart falls on another TNT minecart.

What I Expected To Happen
The TNT minecart falls on the other TNT minecart and they both explode from colliding with each other.

What Actually Happens
Neither of the TNT minecarts explode and they appear to be one (but they aren't. Just break it and you can tell there is two)

Steps to Reproduce
1. Set it up how it looks in the picture, 2 blocks down (or more), place a rail and a minecart with TNT on the rail.
2. Place a block over the hole and place a rail and a TNT minecart on it.
3. Break the block the rail is under.
4. The TNT minecart falls onto the rail and both TNT minecarts do not explode.