The details are not fully clear, but it would seem that the portal filter is the problem. Sum sums related records at the data level. What objects, such as a filtered portal might be present on a layout have no effect on how it evaluates. Both the sum function in the parent record and a summary field defined in the portal's table should both show the same result when placed directly on the overlord layout--a total based on all related records, not a total based on those that pass a particular portal's filter.
To show the total from a filtered portal, Make a copy of that portal--so that it has an identical portal filter and is based on exactly the same table occurrence, reduce it to just one single portal row and put a summary field defined in the portal's table inside that one row portal. This summary field should show a total or other aggregate value based only on the related records that pass through the portal's filter.
Note that there are limitations to this approach that sometimes require using other methods to get the correct subtotal.