I have notice AI Civs can have vassal states without the knowledge of Feudalism in 3.19. It seems to be a bug.
I have attached a screenshot. As you can see, Frederick is vassal of Charlemagne without having Feudalism...
[Edited] Removed the savegame due to quota issues
I have attached a screenshot. As you can see, Frederick is vassal of Charlemagne without having Feudalism...
[Edited] Removed the savegame due to quota issues