WIP RMA transactions taking the Previous cost instead current standard cost

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When we do returns with reference to a sales order, the cost at the time of sale vs cost at the time of return are compared and the delta sits in WIP account (13130). This amount is used as the basis 

We would like system to ignore the delta between original order and current order and just use the current standard cost to do the retrun.

Example: If the SO placed/Shipped with an item standard cost 10$ and later if the standard cost is updated to 15$, then if you place the RMA the remaining balance 5$ will reflect as variance.

Can you please suggest is this a bug?