Hi Maddy,
Kindly see the Tcode carefully--I have mentioned /SAPAPO/CDPSB0 . You can also run this Tcode in background through program /SAPAPO/BACKGROUND_SCHEDULING.
It is NOT CDPS0 (which is meant for Planning Board).
Having said that-- all I am suggesting is can you check in the variant which runs the Production Planning run whether 'With Planning File Entry' Box is checked or not. If its checked then the system does only net change planning.
Remove this tick and then try to run the MRP. Please share the findings.