Playing with the Suebians. They are all Nicene, despite being Arian heretics, which messes up with provincial conversion. Furthermore, going Arian seems to have very little benefits. Their temples are cheaper and quicker to build, but other than that, you are objectively better off going Nicene.
They also acquired Eran Amargar and Vazaarbad ancs. I think they are not correctly assigned to the Barbarian cultural group.
Edit: I forgot to mention, Arianism is missing it's interface icon. Either that or it's corrupted.