I went into a cave and and stacked a bunch of TNT minecarts onto a rail and activated them. The minecarts went off diagonally and started exploding, with the final explosion creating a square/rectangular hole. I attempted this multiple times, and experienced the same thing. It is most prevalent with the new Grimstone block added in this snapshot, but can be seen in stone as well.

This can be recreated by going into a cave or simply beneath the ground, place TNT minecarts on a rail for roughly 30 seconds, and then push the carts forward onto a powered activator rail.