AR Debit Memo Service create DebitMemo only creates the last DebitMemoLine

Subject:
DebitMemoService createDebitMemo only creates the last
DebitMemoLine when multiple line elements are submitted

Service:
https://<host>/fscmService/DebitMemoService?WSDL

Operation:
createDebitMemo

Observed behavior:
The SOAP request contains two DebitMemoLine elements.
The request completes successfully, but only the last
DebitMemoLine is created.

Expected behavior:
Both DebitMemoLine elements should be created under the
same debit memo.

Additional evidence:
Changing LineNumber doesn't affect the behavior.
The service always creates the last XML DebitMemoLine node.