Maybe I need to go check on the behavior now that we've moved to Concur. Previously on AMEX's system seat selection seemed wonky (though I don't remember how, and thus can't confirm the specific behavior you describe) and so I stopped trying to select the seats there. Ever since I always skip selection and just go to the AA website a few minutes later; this is actually good practice anyway because it helps me catch ticketing failures. Sadly those happen a lot when I'm booking inside T-100, as EXP upgrades frequently happen before ticketing and break everything.