Is this during a megawar? Can't remember if they added in safeguards to prevent AI BS of trying to individually declare on lords taking sides, which would really break down the purpose of the megawar if their liege either sat out or joined the enemy and their vassals declared another loyalty, and then just got picked off by a de jure land grabs. Since once the megawar subsides they'll once more come back together anyways so it'd just be wasting troops non-sensibly and potentially defeating the RP where the lord has sword his blade to say the targaryens and marches with them if their own lord just casually stabs them repeatedly.
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