Spurious artifacts in parallel IO or Paraview plotting

Running the following snippet with mpirun -n 4

from dolfin import *


def solve_(mesh):
    RT = FiniteElement("RT", mesh.ufl_cell(), 1)
    DG  = FiniteElement("DG", mesh.ufl_cell(), 0)
    W = FunctionSpace(mesh, RT * DG)
    (sigma, u) = TrialFunctions(W)
    (tau, v) = TestFunctions(W)
    f = Expression("10*exp(-(pow(x[0] - 0.5, 2) + pow(x[1] - 0.5, 2)) / 0.02)",
                   degree=2, mpi_comm=mesh.mpi_comm())
    a = (dot(sigma, tau) + div(tau)*u + div(sigma)*v)*dx
    L = - f*v*dx
    class BoundarySource(Expression):
        def __init__(self, mesh, **kwargs):
            self.mesh = mesh
        def eval_cell(self, values, x, ufc_cell):
            cell = Cell(self.mesh, ufc_cell.index)
            n = cell.normal(ufc_cell.local_facet)
            g = sin(5*x[0])
            values[0] = g*n[0]
            values[1] = g*n[1]
        def value_shape(self):
            return (2,)
    G = BoundarySource(mesh, degree=2)
    def boundary(x):
        return x[1] < DOLFIN_EPS or x[1] > 1.0 - DOLFIN_EPS
    bc = DirichletBC(W.sub(0), G, boundary)
    w = Function(W)
    solve(a == L, w, bc)
    return w


if __name__ == "__main__":
    rank = MPI.rank(mpi_comm_world())

    # Save parellel solution
    mesh = UnitSquareMesh(32, 32)
    w = solve_(mesh)
    XDMFFile(mesh.mpi_comm(), "w4.xdmf").write(w.sub(0))
    File("w4.pvd") << w.sub(0, deepcopy=True)

    # Save sequential solution
    if rank == 0:
        mesh = UnitSquareMesh(mpi_comm_self(), 32, 32)
        w = solve_(mesh)
        XDMFFile(mesh.mpi_comm(), "w0.xdmf").write(w.sub(0))
        File("w0.pvd") << w.sub(0, deepcopy=True)

and comparing parallel and sequential plots in Paraview 5.2.0 shows spurious artifacts on partition boundaries in parallel solution (see attached images). This might be a deficiency in Paraview.

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