No Cashbacks: Discovery wants proof a/c from which they successfully take my premiums (every month) exists!!

More than a year ago I notified Discovery (via e-mail using my Discovery number as reference) that I have changed banks. They made the changes and have been deducting the monthly premium ever since. Only when I download UberEats and it tells me “your cashbacks will be paid into <old-bank-name>” do I discover (and confirmed telephonically with Discovery) that their cash back system still has the old account! No, I did not check each Vitality purchase for cash backs, some of the amounts are small some are large. Surely when a bank account is changed, linked via my central Discovery number, this should be reflected throughout their system? To add insult to injury, they insist that I provide 3 MONTH BANK STATEMENTS AND a copy of my ID (!) to prove the bank account from which they ARE successfully deducting my monthly premium exists!!! How crazy is that? And further – “we will check with the back office to see if they will pay you these outstanding amounts” - IF?! I am speechless. I have posted several compliments to Discovery in the past. How can a company that is run quite successfully in other ways have such a ridiculous system? And then say they will see “IF” the back office will pay me what I am owed?