Using BAPI_PO_CREATE1 to create a PO - MoT from contract is overwriting input

We're using BAPI    BAPI_PO_CREATE1  to create a PO with reference to a conract.

On that contract, we assign a MoT Group - a MoT value that corresponds to several MoT values.

For instance, MoT 91 allows MoT 07, 08, and 09.

When we pass a value to the OIC_MOT field of 07, the PO is trying to create using MoT = 91.

We are using the ExtensionIn structure and are marking the appropriate OIC_MOT = 'X' in the ExtenionInX structure.

any ideas?