Hi Shayan,
Can you please cross-check if both POs are Inter-company?
I believe, one is Inter-company i.e. (Cross company) and another is Intra-company (i.e. within Same company).
In Cross-company --> Incoterm would be copied in PO Header (in Delivery/Invoice Tab) from Vendor Master (based on POrg used in PO --> from Purchasing Data View of Vendor Master). And why Line item Incoterm is BLANK, it's because Vendor Info Record might not exist for that Material & Vendor.
In Same-company PO --> Incoterm field is Hidden at PO Header Level as there is no Vendor here (it's Supplying Plant). So, Incoterm (mandatory field for Delivery) would be determined at Item Level, through a Different Logic i.e. copied from Customer Master (in Shipping Tab), which is determined at Item Level.
Regards,