Edit: This was a confusion due to multiple meanings of the abbreviation CAS in game - both "casualties" and "Cultural Adaptation Score". The wording of the tooltip is still unclear, but what actually happened is: since I unified a different culture zone, as they adopted my culture (Cultural Adaptation Score going towards 100%) they are no longer producing their own unique cultural militia. So first the militancy score plummeted to 0, and now it rises up again - for my own cultures militia.


