Defaults non-mandatory Customer Partner Functions in batch mode

Hello SD Experts,

We are using BAPI to create order in Batch mode in background.

The FM - KOPIERTE_PARTNER_PRUEFEN is defaulting the Z- Customer partner functions even though it is not mandatory. But it sounds weird to me, if the Partner function is mandatory or not passed while BAPI creation. It should not default it. 

I could able see the coding to default the first PF if it is in batch mode, but what would be reason to default the non - mandatory one. 

Thanks for your inputs.

Regards

KK