First National Bank One Account Balance Calculation Error

The FNB One Account balance calculation is a *******. My account started off with a balance of zero at the start of March 2018. During the month I transferred R5300 into the account and withdrew R4500. At the end of the month FNB charged their bond fee of R1900 along with the other sundry charges. The balance should have been plus minus R2500, the statement reflected R4813.00 They say the discrepancy is an overdraft amount not reflected on my statement or online. If you own a FNB One, please check your balance. My telephonic conversations with 3 consultants have yielded no feedback. They don't have a clue on how to calculate the balance and provide answers that drive you to suicide. I am seriously considering dumping them after 17 years.