ELC with dielectric contrasts doesn't work about espresso HOT 5 CLOSED

import numpy as np
import espressomd
from espressomd.electrostatics import P3M, P3M_GPU
from espressomd import electrostatic_extensions

S = espressomd.System()
box_l = 10.0
bl2 = box_l/2.0
elc_gap = 5.0
box_z = box_l+elc_gap
S.box_l = [box_l, box_l, box_z]
S.time_step = 0.001
S.cell_system.skin=0.4

d=0.8
S.part.add(pos = [bl2, bl2, box_l-d], q = 1, fix = [1,1,1])
S.part.add(pos = [bl2, bl2/2.0, bl2], q = -0.5, fix=[1,1,1])
S.part.add(pos = [bl2, bl2*3.0/2.0, bl2], q = -0.5, fix=[1,1,1])
p3m = P3M(bjerrum_length=1.0, accuracy=1e-3)
elc = electrostatic_extensions.ELC(gap_size = elc_gap, maxPWerror = 1e-4, di_mid_top = 1000, di_mid_bot = 1000)
S.actors.add(p3m)
S.actors.add(elc)

n=100
ret = []
for i in range(n):
    z= box_l-d - 1.0*i/n*(box_l-2*d)
    S.part[0].pos = [bl2, bl2, z]
    S.integrator.run(0)
    ret.append([z, S.part[0].f[2]])
np.savetxt("elc.dat", ret)