There's probably a number of forum posts about this now so I'm sorry for the redundancy, but the emperor status and achievement is now bugged on two different Cyrodiil campaigns. Despite a single faction controlling the necessary keeps, it'll show a white block as the reigning emperor and the top AP scorer in the appropriate faction will not have received the achievement.
I'm lucky this hasn't personally affected me as I have the emperor achievement already, but I imagine for a lot of players it is rather frustrating and disappointing. In the past, game breaking bugs that undermine an in-game event have been reason to extend that event, and I'd suggest such a solution might be appropriate here, provided this bug can be resolved at all.
Here is what the campaign screen shows for one of the bugged campaigns: