Change Order does not reduce active encumbrance

We are experiencing an issue when our requisitioners initiate a change order to reduce a PO amount. After the change order is approved by the Buyer, the PO amount is reduced, but the active encumbrance amount remains unchanged. This results in an active encumbrance amount that is greater than the unbilled amount on the PO. Has anyone else experienced this issue? Should change orders not be used to reduce PO amounts?