“Expression worked for 6 previous documents”, but was missing on the 7th. You might need to use object.get(field, defaultValue) to return an object with the right structure when there is no value present.
Also, it looks like the inventoryDetail, when present, contains an array (InventoryAssignmentList/inventoryAssignment). You might need to set up another Mapper with its Mapping Root set to that array to map the individual elements. You might also need another sl.ensureArray to handle a result that only contains one item, since it won’t be an array.