Bad output with Python 3.6.7

[bfadness]

Hi.

I tried to use mgmetis inside a Docker container that uses Python 3.6.7 and NumPy 1.18.1. When I attempt to partition a mesh with 8 cells into 4 parts with the following script,

from mgmetis import metis
import numpy as np
cells = np.array([[0, 1, 4], [0, 3, 4], [1, 2, 5], [1, 4, 5], [3, 4, 7], [3, 6, 7], [4, 5, 8], [4, 7, 8]], dtype = 'int64')
print(cells.dtype)
parts = 4
objval, epart, npart = metis.part_mesh_dual(parts, cells, ncommon = 2)
print('objval:', objval)
print('epart:', epart)

I get a bad partition:

When I run the same script locally with Python 3.8.2 and NumPy 1.18.1, I get a good partition:

Do you know what my problem may be?
Thank you.