Unexpected behavior when using xpress.sum over numpy array slice of xpress variables

I have an application using a 4-D numpy array of xpress variables of which I only want to sum two dimensions leaving a 2-D numpy array. This gives a strange behavior (incorrect) for the first term of each sum of the resulting matrix. Please see here with this simple example summing a 3-D array over only one dimension leaving a 2-D array:

import xpress as xp
import numpy as np
x = xp.vars(5, 5, 5, name = 'x')
sum = np.array([xp.Sum([x[:, :, j] for j in range(5)])], dtype=xp.npexpr)
sum = np.squeeze(sum)
print(sum)

[[[  x(0,0,0) + x(0,0,1) + x(0,0,2) + x(0,0,3) + x(0,0,4)
     x(0,0,1) + x(0,1,1) + x(0,1,2) + x(0,1,3) + x(0,1,4)
     x(0,0,2) + x(0,2,1) + x(0,2,2) + x(0,2,3) + x(0,2,4)
     x(0,0,3) + x(0,3,1) + x(0,3,2) + x(0,3,3) + x(0,3,4)
     x(0,0,4) + x(0,4,1) + x(0,4,2) + x(0,4,3) + x(0,4,4)]
  [  x(0,1,0) + x(1,0,1) + x(1,0,2) + x(1,0,3) + x(1,0,4)
     x(0,1,1) + x(1,1,1) + x(1,1,2) + x(1,1,3) + x(1,1,4)
     x(0,1,2) + x(1,2,1) + x(1,2,2) + x(1,2,3) + x(1,2,4)
     x(0,1,3) + x(1,3,1) + x(1,3,2) + x(1,3,3) + x(1,3,4)
     x(0,1,4) + x(1,4,1) + x(1,4,2) + x(1,4,3) + x(1,4,4)]
  [  x(0,2,0) + x(2,0,1) + x(2,0,2) + x(2,0,3) + x(2,0,4)
     x(0,2,1) + x(2,1,1) + x(2,1,2) + x(2,1,3) + x(2,1,4)
     x(0,2,2) + x(2,2,1) + x(2,2,2) + x(2,2,3) + x(2,2,4)
     x(0,2,3) + x(2,3,1) + x(2,3,2) + x(2,3,3) + x(2,3,4)
     x(0,2,4) + x(2,4,1) + x(2,4,2) + x(2,4,3) + x(2,4,4)]
  [  x(0,3,0) + x(3,0,1) + x(3,0,2) + x(3,0,3) + x(3,0,4)
     x(0,3,1) + x(3,1,1) + x(3,1,2) + x(3,1,3) + x(3,1,4)
     x(0,3,2) + x(3,2,1) + x(3,2,2) + x(3,2,3) + x(3,2,4)
     x(0,3,3) + x(3,3,1) + x(3,3,2) + x(3,3,3) + x(3,3,4)
     x(0,3,4) + x(3,4,1) + x(3,4,2) + x(3,4,3) + x(3,4,4)]
  [  x(0,4,0) + x(4,0,1) + x(4,0,2) + x(4,0,3) + x(4,0,4)
     x(0,4,1) + x(4,1,1) + x(4,1,2) + x(4,1,3) + x(4,1,4)
     x(0,4,2) + x(4,2,1) + x(4,2,2) + x(4,2,3) + x(4,2,4)
     x(0,4,3) + x(4,3,1) + x(4,3,2) + x(4,3,3) + x(4,3,4)
     x(0,4,4) + x(4,4,1) + x(4,4,2) + x(4,4,3) + x(4,4,4)]]]

You an see that the first term in each sum is not correct (only the first one). The progression of first terms should be x(0,0,0), x(0,1,0), x(0,2,0), .... You can see that certain x variables are repeated x(0,0,1), x(0,0,2), x(0,0,3) when know x variables should be repeated at all. Moreover, certain x variables never appear, e.g., x(1,0,0), x(2,0,0), x(1,1,0, x(1,2,0), etc.

Thank you for your help on this issue.