Told there is an outstanding balance when making payment in US

I am not sure if this has happened to anyone. It doesn’t happen everytime but a couple of times I have made a payment with my Monzo the cashier would tell me that some money is still owed. First time it happened was at McDonalds and I gave them cash but when I got home I saw the full amount was taken. Before you say this was someone scamming me it happened again at Arby’s and the lady showed me the receipt that says I still owe a balance of around $5.00. I showed her the transaction in Monzo that it went through and they accepted it but I was just wondering if this has happened to anyone or would know why it happened? I thought initially it was because I had a low balance but I checked and it wasn’t that (also normally it would block a payment if I had a low balance)