Wesbank deducting full installment after full settlement

I had a hire-purchase agreement with WesBank for a vehicle purchased in 2016. I settled the outstanding amount in March 2022, yet, for some reason, my full installment was deducted during April 2022. I tried calling their customer care line (I am holding online for them right now), but after waiting for around 30 minutes then the line dies (and you have to redail).

I left it, as I thought they will rectify their mistake, perhaps I made the settlement too late in March 2022 (even though I done the required work via their website). My April 2022 statement naturally show the settlement, as well as the additional deduction as well as my credit.

However, starting May 2022 they again deducted my full installment. Naturally, I just waste time while waiting for a consultant when I try to call in. As the amount is not trivial, I do not want to wake up until they realize the mistake one day and fix the problem. Naturally, there are no offices that I can visit to rectify this problem, and the response to emails are dismall.