Bank Account closed but refusing to give me a letter as proof

Kati I had a Standard bank account 5 years back, I closed the account due to personal reasons and went to another bank. I have recently made a claim on my medical aid, unfortunately they still had the standard bank account that I have closed and they made a payment to that account, on their side the money has not bounced back and its been over 3weeks now

My medical aid has requested me to get a letter from standard bank as proof that I no longer use the account because they always have difficulties with standard bank accounts that no longer exists because it seems like they give those accounts to someone else. I went to the branch, I found a consultant who just checked and responded that he can't find my details or rather the account doesn't exist but refuses to give me a letter as proof, I no longer use the account but a consultant also said he has no idea who the account belongs to. What I don't understand is that who's fault is it that they can't trace where the money is at, my aim was to get proof that I no longer use the account not me investigating where the money is.