I just received a letter from the IRS saying I have an amount past due from my 2020 taxes, but I paid the full amount in March. Is this a scam?

They said they recently contacted me about past due 2020 taxes, but they did not. I'm confused about what this could be. It appears to be a legitimate IRS address, etc., except the payment address is for Cincinnati, which is not the processing center for my region of the country.