What is the creteria of rate of option according to index

on 01-06-2023 thursday (maturity day) I purchased a put of 43800 bank nifty. by 3.20 the index was 43750 but the rate of the put was 17/- only. So I was forced to sell (square off) it. as per the rule it should have been 50/- but it was 17/- only.
why ?