Change of Order-Type no Partner Re-determination happening SAP MM.

Dear Experts - We have 2 Document Types for PO - NB and ZPRO.

While creating a PO if it is realized that DOC type is to be changed from NB to ZPRO then after doing so PARTNERS are gone.

This does not happen when we create a ZPRO PO and only happens if we change order type while creation of PO from NB to ZPRO.

Note:

1) We have already checked all the Config defined in SAP IMG for Partner Schema and Document Type.

2) Also have already had a look into below SAP Notes.

     459350 FAQ: Partner determination in purchasing

     431351 ME22N Change purchase order, no new partner determination

Pls share some inputs whey this might be happening.

Thanks

Vishal