The indicator Distribute Additional Costs Proportionally on Purchase Orders is automatically set to true (even if you have given false value in the payload against the attribute <CostDistributionIndicator>) while creating the Purchase Order via Web Services.
Reproducing the Issue
You execute the Purchase Order payload either from external system or via SOAP UI. Where in the payload, under <Items> node, you have explicitly given <CostDistributionIndicator>false</CostDistributionIndicator>.
After execution of the payload, still the Distribute Additional Costs Proportionally is marked on Purchase Order for the respective line item.
This is causing because under <Items> node, you have given the following value agiant <BusinessTransactionDocumentItemTypeCode>
Here 004 is for Additional Costs Debit Item.
Change the value against <BusinessTransactionDocumentItemTypeCode> to:
As 002 is for Invoice Items and 003 is for Credit Memo Items.
KBA , AP-SIP , Supplier Invoice Processing , How To