There are two pvd files: they all have a scalar function f(x, y, z) = x + y + z. The first store the mesh with linear elements, and the second store the mesh with second-order Lagrange elements. After setting the opacity to 0.5, the second one shows some internal faces as below. The filter clean to grid does not help. The data is attached.
cmp-cg1-cg2.zip (21.8 KB)