Very sorry for your loss Some Day.
Unfortunately I see no recourse for you. The initial mortgage protection (life) would be based on the mortgage amount and based on that amount decreasing annually (known as term insurance). It is a compulsory insurance. To protect the borrowers in the event of death.
Because you experienced financial difficulties your bank came to an arrangment with you to reduce the mortgage, so no capital was being repaid and therefore your mortgage was not reducing, but your life insurance payout would still be reducing in line with the original terms and conditions of the mortgage. The two products, mortgage and life insurance are not linked. But you must have life insurance to get a home mortgage.
In essence you would have needed to take out a new additional policy to cover the shortfall, or cancel the existing policy and get one to cover the changed circumstances. I doubt very much that you could have afforded an extra cost.
Now whether the bank should have informed you, well I think it should be bank policy but I'm guessing everybody, you and the bank were just focused on saving your house.
It may be something going forward that this insurance aspect was not looked at properly and indeed people cancelled their insurance altogether to save costs.
Despite what I've posted, I think it is worth a try appealing to the your bank. Is the shortfall much?
Another point, and you did not ask it. Would it be better for you to receive the insurance monies and continue to pay the mortgage? That may sometimes be an option, with the banks agreement.
In any case surely the bank is not actually looking to you to stump up the shortfall, rather they just reduce your mortgage amount by the insurance cheque and you continue paying a lower mortgage?