An Accessorial cost attached to Buy shipment comes to several Sell shipments after Recalculation cos

Dear Sirs,
Our problem is:
An Accessorial cost attached to Buy shipment comes to several Sell shipments after Recalculation costs.

We have an order with several legs.
We created shipments for both parties (Buy and Sell).
We have some addition accessorial costs.
We could count them before.
They appear unexpected.
This cost belongs only for one leg.
We attached this cost to one shipment in a system.
We marked “Cost Fixed” and “Process As Flow Through”.
Then we save this cost and shipment.
The next step is to make a recalculation on the Sell side.
We did it.
But this additional cost is attached to everyone Sell shipment.