https://userapps.support.sap.com/sap/support/knowledge/en/3502314
3502314 - COM_KBV3-VBELN is not filled during output processing from COM_VBUK | SAP Knowledge Base...
During output processing COM_KBV3-VBELN is not filled from COM_VBUK, which was not the case in ECC in form KOMKBV3_FILL (include LVCOMU05).