When a line item is included in a sum the Is Summary automatically enabled. This can be disabled manually in blueprint mode.
When adding additional line items to that sum, the Is Summary automatically re-enabled.
This default behavior has some undesirable impacts for exports where summary items are omitted.
A quality of life improvement would be that Is Summary for a line item should not be re-enabled if it was previously disabled as this is more intuitive. Workarounds are possible for this behavior as omitting summaries from an export is not necessary with proper filtering/showing of line items.