(Is this a bug?)Auto-Liberation of cities on conquest.

[Zeek]

Using standard BTS rules.
Setup:
Egypt asks me to help them in their war against Zulu and I accept. After a few turns, Zulu conquers a few Egyptian cities while i gather my forces. A few more turns pass and Egypt asks to be my vassal, I accept. I send a stack in to punish the zulu. In the attached save, my stack is ready to attack thebes (which the zulu conquered after i declared war on them, but before egypt became my vassal). Use the stack to conquer Thebes, it will automatically be turned over to Egypt. I don't know if this is intentional or not, but if it is, I can't find it documented anywhere.

Edit:
After I conquered Thebes, I went on to Heliopolis, another city which the zulu conquered from the Egyptians, and this time it wasn't auto-liberated. Perhaps it's a culture level issue? E

 

[H&K]

No its not a bug.
If you take a city that conciders itself to be of your vassels nationality it considers your taking of it to be Liberating it for your vassel.

If this doesnt happen, then that city generaly doesnt concider itself of your vassles nationality. Because either it was held so long its nationality changed, or (More normaly), it was a city your vassle took from someone else, and never concidered itself your vassles nationality