@vincentt – since this is a MBS, the cashflow repayment include schedule principal repayments, hence FV=0. So at the end of life of an MBS, it should be expected that the whole mortgage is repaid in full.
The maturity of MBS is uncertain as there are prepayment risks, i.e. people prepaying more than the minimum amount each month, hence shortening the life of the mortgage.
It’s always related to bond equivalent yield, so make sure you get these 2 concepts right.