I went down to the flat area. As soon as I reached it, I started to dig.
First the surface got slightly deformed. I kept holding the mouse but the pick didn't dug deeper. I could hear the pick hitting the surface.
I thought that maybe the surface is just a thin layer above an indestructible limit.
So I moved a few steps away and tried again. Then I could dig deeper.
I went back to the original location and I was able to dig there too.
I could not reproduce the problem.