This is exactly what happened to me! (The serial mismatch issue). I have been on the phone with Apple and citizens literally all day. I wish someone could have explained to me the issue happened with the replacement device I exchanged last year. Ugh. No one seemed to know why I was getting the mismatch SSN error message. They told me to just start a new loan instead. Annoying because now I have a new hard pull on my credit.
That's horrible. You think they'd have fixed it by now. This was over a year ago for me, so I can't remember which department at Apple ended up fixing it. They had to get the Apple Care + transferred over first, if I remember correctly, and then it was escalated up to a senior engineer. I would recommend emailing Tim Cook so someone on the executive support team can look into it as well. I do not recommend signing up for a new loan as it will pull another hard inquiry on your credit report vs a soft for a renewal. All I know for certain is that it is on Apple's end, not Citizens One.