vtkUnstructuredGrid::DeepCopy with faces
Description: If a vtkUnstructuredGrid object contains faces (because some/all cells are of type VTK_POLYHEDRON), the faces data structures are not copied with DeepCopy().
Workaround: before calling DeepCopy() from an unstructured grid with faces, call Allocate() and InitializeFacesRepresentation(). The reason is that Faces and FacesLocation will only be copied if they exist already in the target unstructured grid, which will be done by the second call, and the first is required because otherwise Types does not exist which is assumed in the second call.
Note: this report is referring to the VTK version that is included in ParaView 5.8.0 official release.
See also: VTK Discourse