Dear Olal
the "incompleteness" schema seems to be wrong. This schema is "sales" schema (in combinaiton with DG). An the top your DG schemas are in my opinion wrong. Normally nobody ! wil check for mixed loading on sales level but only on delivery (or shipment/transport level); if a check is done on sales you should get a "warning" in DG and not an error
What I do not understand 100%: in the automatic process it does not work but in manual process it does?
Therefore: please explain you DG schemas used and your inceompletness set up (link of SAP SD and EHS DG)
C.,B:
PS: check may be Dangerous Goods Checks in Sales Documents - Dangerous Goods Management (EHS-DGP) - SAP Library
deeper
PPS: e.g. Define Incompleteness Procedures - Dangerous Goods Management - SAP Library