I have already posted this, but it was quickly miss understood to be missing cobblestone. The cobblestone is there, likely the cobblestone was generated after the gravel and therefore generates under the fallen gravel.
I was flying about a new world (Seed -8732934775646344018) when I came across a village (-298 599) that had generated part of it's path in a cave, even with the new cobblestone 1 layer underneath the gravel.