If you refer to
Giving different solutions in parallel, this is not suprising, as when the code is executed with MPI the mesh is distributed, which in turn means that the solution is distributed across the requested processes. No single process store the full solution.
Some time ago I posted how to gather solutions: Gather solutions in parallel in FEniCSX - #2 by dokken
and an explanation of parallel distribution at
Parallel Computations with Dolfinx using MPI — MPI tutorial