I have almost the same problem. I have an Expression that is the multiplication of two parts (Part A * part B). When I make the sum of my own for the two parts, and then multiply both TOTALS it is correct. But it is not the correct total for the pivot grid.
This is the Expression
(Sum(STOCK_AL_CIERRE_AJU) - Sum(VENTA_AJU) + Sum(CAJAS)) // PART A
