This is likely due to tag backlinking being enabled in FlexReports.
If there is a cost difference for an AWS resource it is likely due to the tag values changing.
For example, if a resource has a tag value of “NULL“ and also a tag value ”XXXX” on same day, FlexReports we pick up the latest tag value for a resource. The new value will be updated in the Current Month populate back to previous months.
In the above situation when the line item is null as latest entry it is applying the tag value NULL for this resource to previous months and affecting costs appropriately