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.
SAP Business ByDesign
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 against <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