@Lea CC Hi, normal assembly items and inventory items get added just fine. The problem is just with Phantom-items. I do not set anything to that plannedissuedate on those either.

objRecord.selectNewLine({ sublistId: "item" });
objRecord.setCurrentSublistValue({ sublistId: 'item', fieldId: 'item', value: '1234' }); //Phantom item
objRecord.setCurrentSublistValue({ sublistId: 'item', fieldId: 'quantity', value: 2 }); //Quantity
objRecord.commitLine({ sublistId: 'item' });
var newObjRecordId = objRecord.save({ enableSourcing: true, ignoreMandatoryFields: false });