charged while gym contract had been cancelled

In 2021, I was on a month-to-month contract with Virgin Active. I visited the Boulders branch to inform them that I would no longer be using their gym services as I was relocating to Limpopo, where there is no Virgin Active facility. At the time, I was assured that there was no payment required from my end, as the binding contract I had initially entered into had expired. They confirmed that no further action was needed on my part.

Yesterday, on the 10th of April, I visited the Virgin Active branch in Carlswald to open a new membership, as I have returned to Gauteng. To my surprise, I was informed that I owe R6,000. I queried how this amount could possibly be owed, given that I was on a month-to-month contract when I left and had canceled my membership at the Boulders branch.

The staff member assisting me mentioned that I could pay half of the amount as a settlement. I declined, saying that I would rather consider Planet Fitness if that were the case. I am seeking to understand how and why I accumulated such an amount. Is this standard practice, or is this a way to exploit clients? I require a detailed explanation.