K_africa should be de jure Byzantium in 769. Carthage was lost in 698, less than 100 years prior.
Problem with de jure empires is that:
A: Kingdoms can´t pass out of them unless they switch to another empire, so unless someone controlls all of de jure Africa for 100 years, it will always be considered de jure part of ERE, even if they haven´t been there in 500 years.
B: Empires tend to take back their de jure lands, but not much else. So, if your suggestion passes, we would see ERE taking Africa, but not Egypt, Jerusalem, Italy or Syria, even though all of those are more important than Africa. You would just end up with Roman enclave in Africa. And problem connected to this is that those lands are extremely hard to take back, so this enclave will remain there for hundreds of years.