different result after scenetostring

Hi:
I noticed that the results of my simulation is different if I use 'scenetostring' and 'stringtoscene' (I also tested save and load). I generated a periotric cell and change to strain from (0,0,0,0,0,0,0,0,0) to (0,0,0,0,0,0,0,0,-0.2) to (0,0,0,0,0,0,0,0,-0.1).
Here are my codes:

from libyade import yade
from yade import*
from yade import pack, Vector3, Vector3, utils
from minieigen import Vector3, Matrix3, Matrix6

def randomPeriPack(radius,initSize,seed):
 O.switchScene(); O.resetThisScene()
 sp=pack.SpherePack()
 O.periodic=True
 O.cell.setBox(initSize)
 sp.makeCloud(Vector3().Zero,O.cell.refSize,radius,0.,-1,True,seed=seed)
 O.engines=[ForceResetter(),InsertionSortCollider([Bo1_Sphere_Aabb()],verletDist=.05*radius),InteractionLoop([Ig2_Sphere_Sphere_ScGeom()],[Ip2_FrictMat_FrictMat_FrictPhys()],[Law2_ScGeom_FrictPhys_CundallStrack()]),PeriIsoCompressor(charLen=2*radius,stresses=[-100e9,-1e8],maxUnbalanced=1e-2,doneHook='O.pause();',globalUpdateInt=20,keepProportions=True),NewtonIntegrator(damping=.8)]
 O.materials.append(FrictMat(young=30e9,frictionAngle=.1,poisson=.3,density=1e3))
 for s in sp: O.bodies.append(utils.sphere(s[0],s[1]))
 O.dt=utils.PWaveTimeStep()
 O.timingEnabled=True
 O.run(); O.wait()
 for b in O.bodies: b.state.pos = O.cell.wrap(b.state.pos)
 ret=pack.SpherePack()
 ret.fromSimulation()
 O.switchScene()
 return ret

young = 4e6
O.materials.append(CpmMat(young=young,poisson=.2,epsCrackOnset=1e100,sigmaT=1e100,relDuctility=2))
sp = randomPeriPack(.01,.1*Vector3.Ones,10)
sp.toSimulation()
O.engines = [
 ForceResetter(),
 InsertionSortCollider([Bo1_Sphere_Aabb(aabbEnlargeFactor=1.5,label='is2aabb')],allowBiggerThanPeriod=True),
 InteractionLoop(
  [Ig2_Sphere_Sphere_ScGeom(interactionDetectionFactor=1.5,label='ss2d3dg')],
  [Ip2_CpmMat_CpmMat_CpmPhys()],
  [Law2_ScGeom_CpmPhys_Cpm()]),
 NewtonIntegrator(damping=.1),
]
O.dt = 0.
O.step()
O.dt = 0.0002
is2aabb.aabbEnlargeFactor = 1.
ss2d3dg.interactionDetectionFactor = 1.

Strain1 = Matrix3(0,0,0,0,0,0,0,0,-0.2)
Strain2 = Matrix3(0,0,0,0,0,0,0,0,-0.1)
I = Matrix3.Identity

trsf1 = Strain1+I
trsf2 = Strain2+I
v1 = (trsf1-O.cell.trsf)/(500*O.dt)
O.cell.velGrad = v1
O.run(500,True)

print(O.cell.trsf)

scene = O.sceneToString() ####!!!!!
O.stringToScene(scene) ####!!!!!

v2 = (trsf2-O.cell.trsf)/(500*O.dt)
O.cell.velGrad = v2
O.run(500,True)

print(O.cell.trsf)
print(utils.getStress())

If you run the script, I think the result of stress[8] should be -567126, and if you delete the scenetostring and stringtoscene, the result is -733878. My yade version is 2017.01a. Maybe my yade version is relatively old??

Yours,
Joe