Reversal of claims after 2 years

Received a Amount Due to Discovery Health Medical Scheme letter on 23 November 2018 stating that I owe R1037.06. I queried this and was told that Discovery reversed payments for a Doctor that was made on 13 Oct 2016 and 3 Nov 2016. It took 2 years and now I have to pay them the money. Surely the Auditors they use are ***********! They are strict about clients submitting claims within 3 months but yet they can take 2 years to inform you about reversals. I am extremely disappointed and don't think I should be held liable for their incompetence.