Hi Srinivasa,
The MIRO does not have any inventory posting. Generally system will generate inventory postings when there difference between MIRO price and PO price. If the price is same then it won't generate any posting. So system is behaving correctly. Try to test it by posting the MIRO with price difference. After post it, then again run the FIFO valuation and check the impact.
Hope it clears now.
Regards,
Mukthar