IPN

Chase has been sent the IPN for the payoff of mortgage. They signed it "received" and then sent back that original signed document. A month later the account still shows the payoff amount and we feel to make the monthly payments until the account shows "0" or "paid off". We assume that this particular banking process make take 30-90 days before the city, etc. signs off. Is this correct? What is more confusing is that they signed "received" and still sent us the original. Is this the normal process?