Just to update. Playing another Romani campaign and the same thing happened. Marcus Atilius Regulus - who I set as potential heir - becomes heir on the death of Dentatus. No matter who I set as potential faction heir following Dentatus' death, once Valerius Maximus dies Marcus Atilius Regulus (the eponymous son) becomes faction heir, bypassing the potential faction heir. Perhaps there's a hangover from 'first born'...or whatever it's called affecting the choice of heir?