Hi Bhargavi.
in AFS scenary you should use lot size J... and not standard MB and so on beacuse you have category and size to manage in the MRP run.
You can check in J_3ADBBA table all details of MRP data.
Another soluction could be use the following Badi:
/AFS/MRP_PURREQ_CHANGE
for change vendor and others data during creation of pruchase requistion with our logic.
I hope was useful for you
Let me know if i solved your problem.
Daniele