Volume mesh contains non-tetrahedron cells

[hadiiiil]

Hi, I encountered this problem while trying to generate a volume mesh from a 3D array, which the resulted mesh had tetrahedron cells but also non-tetrahedron cells which is not good for the finite element analysis. I have tried to generate a surface mesh first and then use volume-from-surface-mesh but I encountered the same problem again which is non-tetrahedron cells exist.
I tried to use delaunay_filter = vtk.vtkDelaunay3D() to remove the non-tetrahedron cells but the result was that the mesh in not uniform or isotropic.

My question is, is there a way to fix this problem? and why does it raise? And if I can't avoid this issue how can I refine the mesh after removing the non-tetrahedron cells.