US citizen cannot check in online Dublin to US - why might this be?

I'm a US citizen travelling to the US on their US passport on a round-trip ticket with a UK citizen on a UK passport. We're going London Heathrow to Dublin to our US destination city. Check-in opened a couple of hours ago. My travelling companion can check in online for both legs of the journey. I can check in for Heathrow to Dublin, but it's saying I have to check in in person for Dublin to USA. Aer Lingus says they have no information on why this is, but that it might be because we booked using a third party site and that can sometimes happen in those cases for one passenger but not another.

Has this happened to anyone else? Given the situation in the US I'm a little concerned. We have visited the US together many times before and I don't remember ever having to check in in person. I have no criminal record nor any involvement in criminal/nefarious activity, but I gave a conference presentation in Europe earlier this year that could be construed as regime-critical indirectly.

Paperwork-wise there's nothing unusual that I can think of except that my US passport was renewed only about a month ago and this is the first time I've travelled on it.