Non-reproducible Polyhedra contact force

Considering a Polyhedra description of two fixed contacting spheres (see image below), the measured contact force (even its orientation) is not reproducible.

Illustration

Script (also attached as poly4.py )

from yade import polyhedra_utils
toZplus = toZminus = orthZ = 0 # counter initialization
for i in range(4):
    print('Run',i+1,'/ 4, please wait a couple of seconds')
    O.reset()
    O.materials.append(PolyhedraMat())
    for z in [0,1.7]: O.bodies.append(polyhedra_utils.polyhedralBall(radius = 1,N = 200,material = O.materials[0],center = (0,0,z)))
    for b in O.bodies: b.dynamic = False
    O.engines=[ForceResetter()
               ,InsertionSortCollider([Bo1_Polyhedra_Aabb()],verletDist=0)
               ,InteractionLoop(
                   [Ig2_Polyhedra_Polyhedra_PolyhedraGeom()],
                   [Ip2_PolyhedraMat_PolyhedraMat_PolyhedraPhys()],
                   [Law2_PolyhedraGeom_PolyhedraPhys_Volumetric()]
               )
               ,NewtonIntegrator()
    ]
    O.step()
    fz = O.forces.f(1)[2] # on top body
    if fz > 0: toZplus+=1
    elif fz < 0: toZminus+=1
    elif fz == 0: orthZ +=1
print('\nAfter 4 runs, we detected on top body:\n-',toZplus,'times an upwards force to z+ (should be 4 times)\n-',toZminus,'times a downwards force to z- (should be 0)\n-',orthZ,'times a horizontal force (should be 0)')

Possible output with yadedaily and cgal 5.0.2 (not reproducible itself, please consider to re-run if you get the expected output)

Welcome to Yade 20210905-589402eb5b41focal1

[.. Note an unknown warning ]

After 4 runs, we detected on top body:

• 3 times an upwards force to z+ (should be 4 times)
• 1 times a downwards force to z- (should be 0)
• 0 times a horizontal force (should be 0)

See #242

Edit: mentioning CGAL version