Overdrawn account

I have received two notifications of money which has been deducted off my account, both occurred in January 2020. At the time of purchase I had more than enough funds in my account to afford it (otherwise they'd have never gone through) and now it has resulted in my account being overdrawn. I am shocked at FNB's inability to reserve funds in my account when I make a purchase, I mean what the actual hell is going on? Are we living in the past? I left Nedbank for the exact same reason, it's almost as if they wait for me to have no funds in my account and suddenly all my transactions went off. I have never had this problem before with FNB but this does not do much to inspire confidence. Your inability to actually reserve funds when they're available and I make a purchase is not my problem at all. Now I am stuck with owing over R200 and I am a student who needs every cent of my money. This is shameful and quite frankly unacceptable. This will be the reason I leave FNB as well. God knows how it seems to be so impossible to just keep money aside when a person buys something!!