Question about credit card interest charges

I have a question about how credit card interest charges work.

Long story short, my checking account ran low on cash at the end of July and I was expecting several electronic debits for bills. I transferred a couple of hundred dollars from my credit card to my checking account as a cash advance back to hold me over a couple of days while I did an ACH transaction from a high interest savings account at a credit union in a former city I used to live in.

I repaid the money a couple days later and then paid the card off in full as part of my normal payment cycle toward the end of August like I always do. I paid 70 some cents in interest on the cash advance, which was expected.

I paid my card off in full again toward the end of September, but I just noticed that on September 30 I was assessed 1 cent interest charge on a 69 cent average daily balance cash advance. I'm sitting here thinking to myself, "what the hell?"

I plan on calling the bank Monday and asking what's up, but can someone tell me what I might expect?