Lucknow Nagar Nigam — House tax bill

Sir,
The House Tax office Lucknow nagar nigam issued bills every year upto 2020--21. and i paid according to bill amount in time every year. The Nagar Nigam site is showing No arrears due for the F.Y 2020--21 against my name and address and house number. . Now in January 2021 Nagar Nigam Lucknow Has issued notice of heavy increase in house Tax backdated to 1/4/2019. and asking to pay the arreears. Does that mean that Nagar Niagm Lucknow issued WRONG BILLS and if YES why should customer suffer for Nagar Niagm office Mistake. Please advise.