https://fat.gfycat.com/DeficientIlliterateCollie.webm
Here is how it works usually in 1.12

Here is how it works in 17w48a
https://giant.gfycat.com/ElementaryJitteryGuernseycow.webm

I was testing my old reliable TNT cannon, which uses stacked TNT minecarts as propulsion. I noticed that the TNT minecarts do not function like usual. Usually when they get pushed when stacked they stay sticked together like a single entity. In the snapshots however they separate when pushed when they are stacked, making them explode in your face.

This completely breaks super compact TNT minecart cannons as we know it, which are very versatile and very very compact due to the fact that you can stack TNT minecarts which acts as an propulsion charge. You can use it for everything ranging from regular TNT cannons to Anti-air cannons to absurdly good elytra launchers.

https://www.youtube.com/watch?v=EhtqyZey3g0

https://giant.gfycat.com/WateryLegalDassie.webm

Stacked TNT minecart behaviour in 1.12.2
https://gfycat.com/gifs/detail/SelfassuredGlaringLeafcutterant

Stacked TNT minecart behaviour in 17w49a
https://gfycat.com/gifs/detail/SickCaringAlbertosaurus

Stacked TNT minecart behaviour when accelerated by a rail and tossed off a cliff in 1.12.2
https://gfycat.com/gifs/detail/ZestyCalmHuman

Stacked TNT minecart behaviour when accelerated by a rail and tossed off a cliff in 17w49a
https://gfycat.com/gifs/detail/UntidyOldfashionedAlbatross

It also breaks the stacked TNT minecart bombs. Quite useful for clearing large areas or even tunneling wider tunnels than what the slime block tunnel bores are capable of.

This leaves us with the regular TNT as propulsion for cannons, which are very very lackluster after 1.8. They can barely propel a primed TNT unlike TNT before 1.8, likely a nerf or unintended change.

The new change seems to remove a LOT of possibilities while adding very few new ones. It'd be a great loss for people who like messing around with cannons and missiles. Without the 1.12 minecart stacking mechanic TNT minecart cannons will no longer be possible same with TNT minecart Anti-air cannons, TNT instant landmines, Neither will TNT minecart anti-air cannons, or TNT minecart tunnelers and TNT minecart elytra launchers.