It clearly seems like an IDB to me, but it wasn’t quite clear to me from the above - did the OP check on the app to manage their booking? Last time it happened to me, I immediately got online to check my my new flight, and even changed my seat. (And then set an EF seat alert; yes, I am that paranoid about babysitting my reservations.) If I couldn’t get a seat, I’d assume something was wrong.