I am using Fenicsx 0.8, and try to use VTXWriter to write output files. However, it can only write correct output for one function but constant output file for the other function. I tried to use XDMFFile, both functions can be outputted correctly.

sample code:

```
V= fem.functionspace(mesh = domain, element=("Lagrange",1, (3, ) ) )
u = fem.Function(V)
with XDMFFile(MPI.COMM_WORLD, 'test_u.xdmf', "w") as xdmf:
xdmf.write_mesh(domain)
s_space= fem.functionspace(mesh=domain, element=("DG", 0, (3, 3)))
s_sol = fem.Function(s_space)
s_sol.name = "S"
vtx_u = io.VTXWriter(MPI.COMM_WORLD, 'vtx_test_u.bp', u)
vtx_s = io.VTXWriter(MPI.COMM_WORLD, 'vtx_test_s.bp', s_sol)
for n in range(1, 10):
# do things
# u is solved using Newton solver
num_its, converged = solver.solve(u)
# Then get s using projection,
s_testFun = ufl.TestFunction(s_space)
s_trialFun = ufl.TrialFunction(s_space)
a_s = ufl.inner(s_trialFun , s_testFun ) * dx
L_s = ufl.inner(PK1, s_testFun ) * dx
s_problem= LinearProblem(a_s, L_s)
s_sol_this = s_problem.solve()
# VTXwriter gives the constant s over time, same as the first step
# but u output is correct
s_sol.x.array[:] = s_sol_this.x.array
vtx_u.write(n)
vtx_s.write(n)
# xdmf gives correct u and s
xdmf.write_function(u, t=n)
xdmf.write_function(s_sol, t=n)
# xdmf.write_function(s_sol_this, t=n) also work
```

Thanks.