Line Items not getting copied in Next Revision

Hi,

While revising , a write record is happening on S_ORDER_POSTN it is trying to write the record with Order_Id of the current Order, Position_Id of the current order but Order_num as 1 , which is causing User Key violation issue as on our table level for table S_ORDER_POSTN there are two user keys : ORDER_ID and POSTN_ID.
This is happening for some specific position and happening only on production .
if we check on some other environment which is copy of production , with the same Position, issue is not getting replicated.

What can be cause of such user key error.