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While the events triggers and all that, it doesn't seem like the Sarnori building chain has been implemented in the event, so the Success! event doesn't actually accomplish anything.
This is still a problem in the new version, except now the Success is always attempting to build the first/lowest level building regardless of the level of the castle you already have in the holding, so it doesn't get updated. I've checked, and the event still works for other cultures, it's just the Sarnori buildings that are problematic.