Restricting Commitment Splitting - R42520


We are using R42520 to allocate lots to Sales orders line type S. We have set Item Branch Commitment Date Method to 1 as we want to use oldest lots first.
Our product is such that we never want to mix lots, but are happy to pick the same lot from multiple locations.
Is there a way we can set the data such that R42520 will split between locations for the same lot, but not split between lots for the same item when committing?

We have set Customer Master Billing settings to avoid partial shipping which prevents splitting between a lot and back order.
For info: We have lot control switched on and do not use advanced warehouse management.

Strangely, I have found that R42520 seems to handle lot commitments exactly as we wish when the sales order cost is set to zero. As soon as we introduce a line value the UBE splits commitments strictly by oldest lot first.

Ideas gratefully received.