Stacking tnt minecarts and detonating them acts differently than previous versions (Don't know if it's intended or not because there isn't any specification on this)
– What i was expecting (and what happens in versions BEFORE 1.12):
When you stack multiple tnt minecarts into a single block and then detonate them by making them crash into a block, all the tnt minecarts are supposed to blow up ALL AT ONCE.
– What is happening (ONLY happens in 1.12):
When you stack multiple tnt minecarts into a single block and then detonate them by making them crash into a block, the tnt minecarts each detonate 1 by 1.
To create this :

• Place a rail
• Stack a couple of tnt minecarts on that rail
• Give the carts a slight push
• Then make them crash on a block
• Test on 1.12 and other version (other than 1.12)
• Compare