In general, the filtering applied to a DataItem (a Table in this case) is applied to its entire data set, and any expression inside the aforementioned DataItem can have access only to subsets of the filtered data. If your Sum function does not respect that, I suspect that it is defined outside the scope of the Table. Could you verify if this is the case?
Speaking of "scope", it is important to mention that the result of an expression is highly dependent on the scope it is defined in. For example, if the Sum function is used in the scope of a particular Table group (more details about groups), it will sum only the values in said group, instead of summing the values inside the entire Table.
I am including a short demo report that illustrates these points.
If the issue persists, you could send a report sample to me so that I can investigate it locally.
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