I am neighbouring the Kingdom of Navarra. Some of their holdings have mixed ownership. As you can see in this screenshot, the castle belongs to Navarra, while the temple and the city belongs to the Emirate of Galicia.
If you hover over the Emirate of Galicia, the tooltip says:
De Jure: Emirate of Galicia.
The County of Santiago is traditionally considered part of the Emirate of Galicia. Emir Rasiq of Portucale is the current duke, who will strive to regain actual control of Santiago.
This is not the only example – many holdings in the Iberian peninsula have mixed ownership. As you can imagine, this makes it confusing to create an overview over allies and enemies in the region.
How is this possible? Is it a bug? I am playing an older version of the game – 2.0.4.