Adjustment Code of Shipment Cost not copied to Invoice Line Items while Auto Approve

I have a Shipment ->Shipment Cost Line Item -> Adjustment Code value. When an Auto Approve Rule is applied, the GL Code details along with others Shipment Cost line Item details are copied over to Invoice Line Items respectively. But Adjustment Reason Code does not get copied over to Invoice Line Item (while Auto Approve Invoice is being done). Why is that so? Should it not get copied too? Please advice.

Thanks.