Hi Jolin.
What kindo of order is involved ? PP Order or CO Production Order?
The default valuation is set in in tcode OMWC. It depends on the valuation type, Valuation Category and Plant. The tcode is a bit complicated to understand, if your MM partner customize the split valuation ask him.
If you work with PP Order, in the tab "Goods Receips" you can see or modify the valuation type. In the customizing there is a flag that allow modify or dont modify this stuff.
Wish this clarify.
Arturo.