TNT not exploding in a group

Today I was doing some mining with my brother when this bug happened.
What I expected to happen was the TNT to explode in one big group since I had put about 40 of them in a cube. What actually happened was the TNT exploded individually so we had to end up igniting the TNT one by one.

Steps to Reproduce:
1. Make a large block of TNT
2. Ignite one TNT with flint and steel