AWS connection cannot remove the inventory device records that are linked to assets. This behavior is applied to all types of inventories. There is a rule in FNMS that "Where there is an computer asset record, there should be an inventory corresponding to that record". We explain the rule as following: Any inventory does not show up in several reports, we removed it. For inventory that linked asset record, even when the inventory may not show up in several imports, we know there is a machine and there is an inventory, it just can't report itself, so we don't remove the inventory record.
An enhancement ticket FNMS-69828 has been raised for this. The enhancement should allow FNMS automatically remove terminated instances regardless of having asset records linked to them or not.