Regarding WORDSNET-19645, it is to update you that after further investigation we have found that the SUM fields are actually updating correctly, but ,the field contains a space Run following the FieldStart node which is colored white. Aspose.Words applies white to the result of the field thus making it invisible (and it only becomes visible when field is selected). And MS Word does not.
So, in the meantime while you are waiting for a fix, please apply a color other than white to the first space character after the FieldStart node (looks as a white circle when the field is selected and hidden formatting is on):
Or, to make things simpler, just select the whole field (or containing cell) and go to Font - Font Color then apply anything but white (Automatic would be fine).
We will inform you via this thread as soon as this issue will be resolved in future.