Blocks are being broken, despite being protected by blast resistant blocks, such as obsidian.

Steps to reproduce the issue:

1. Place x-amount of blocks, such as oak planks, or glass.
2. Surround the blocks with obsidian against all exposed faces of the blocks you are aiming to protect.
3. Place tnt on any side of the obsidian, and prime it with a flint and steel.

My best educated guess would be that the explosion rays of TNT get through the connecting coordinates of 2 connecting blocks; seeping through the cracks, if you will.

I've provided a gif of a small recording for your viewing pleasure.