Momentum refuses to pay for an appointment and visit made when membership was valid

I was only due for an optometrist after August 2021, my appointment date and visit was 26 August 2021, 5 days before the expiry of my membership. Only to be called in October 2021 that I have to pay for the services by myself. I visited the optometrist during a period when my membership was valid and Momentum confirmed this by authorizing this booking. Please Momentum, why now are you not paying for the bill, I am so disappointed.