FNB's charging interest by playing cheap trick

Good day Friends, I would like to share really bad experience from FNB recently. I am FNB credit card account holder and I clear my full outstanding balance every month on before payment due date and I can prove the same by showing at least last 5 years bank statements. I think last month or before last month minimum amount was debited from cheque account and I decided this month to pay by deducting minimum amount due and paid remaining and this month FNB haven't debited the minimum amount due from my cheque account and charged the interest on whole outstanding amount. I know how credit card works, just to avoid double payment I have deducted. I have paid whole amount and its not a big matter for me to include the minimum amount in the full amount. I hope you guys have understand my point. Is this the tricks fnb using to charge interest and make money? Is this the time to move to discovery bank? Please advise with your comments and if you experience same issue with fnb. If FNB doesnt refund me, I am planning to switch to discovery or capitec. Thank you for your time on reading this mates.