Dear Nabin,
Kindly check carefully that you assign the right GL to right valuation class in OBYC with GBB-VBR, it's the standard behavior of system i.e. what ever GL is assign to valuation class of Account Assignment K with ACCT modification VBR, only assigned GL would come in PO, I dame sure that you are missing some thing else no way to come GL from GBB-INV.
Snap shots are given below
Best Regards
Mohsin Abbasi