Hello Jie,
if i right assuming your query then PR->PO= Commitment should be created.
but GRN by MIGO=Statistical cost should not create once MIGO if right then actual cost would be 0.
you can activate commitment by OKKP transaction.
And you want to cost should not be statistical after MIGO right?
here solution is cost never statistical type until account assignment object not statistical type this is my assumption.
if this is the issue then check G/L account from the PO. then check that G/L account, whether it is a B/S account or P&L account? I think you have wrongly assigned P&L account.
Regards,
Sanjeev