Hmm I think you're right... when I look in the "Clans" section of the encyclopedia, that clan does show up under the Vlandia filter despite all its members being Empire. Seems logically inconsistent to me, but at least it's
an explanation
.
The annoying thing is that there's nothing on the individual clan's encyclopedia page that mentions the clan culture. I'm pretty sure this doesn't come up in the conversation where you make them a vassal either?