Hi
Some time system won't Read the sold to party and ship to party codes from Billing document details due some programming Bug.
Kindly apply the Below mentioned SAP Notes and test the cycle.
SAP Note : 1444333-Sold to party not updated for outgoing EI Create Background
SAP Note: 1656519 - Sold to party not updated correctly while EI creation -VF01
I hope it will solve your issue after applying the SAP Notes.