Route code not updated on all lines of Purchase order

Hi,

We came across one issue where user initially raised purchase order for stocked item (line type S) on which approval route code updated correctly on that line. After that user added 5-6 lines of non stock item (line type J) under the same order but route code not updated on those lines. User expecting same route code on these lines also. I checked, PO order type for stock and non stock item is OP and route code is also same.

Can you please suggest if we can add both S and J items under one order with same order type and route code? If yes then what is the possible cause to not update route code on J type lines? If no, then what set up is required to do the same?