Dear Ashok,
I addition to my reply below. I have tested reverse scenerio.To check whether system is taking stock type from PO.
As you have made the assumption that
When you perform GRN system defaults the stock type from PO.
I would like to comment on it.
Consider this Case: I have activated inspection type-01 in QM view, but deactivated 'Post to inspection type' indicator. Created PO in which stock type is 'Unrestricted'. After creating PO, i have activated 'Post to Inspection' indicator. Now while creating GRN, stock type is 'Quality Inspection' that means in this case the system is checking from MMR and not from PO.
Sir, Correct me if i am wrong. Also, Kindly provide the solution or should i approach to SAP.