Hi LI.
This is not a "Bug" in the system. When you resubmit a purchase requisition, the original workflow instance gets cancelled and a new one is auto-created and submitted.
I'm not 100% sure why MS decided to do this on Pos, but I suspect its to enforce that the whole workflow process is followed from start to finish on resubmit, rather than just going back to the step that had the original change request.