I've been sieging Casterly rock and on multiple occasions Archers would spawn in completely inaccessible areas.
Since the AI seems to be able to see throgh walls, this will mess up attackers as they will stop their advance and stare at a spot on a wall instead of moving inside the castle.
https://imgur.com/a/HSslF (https://imgur.com/a/HSslF)
In this case 2 Archers are inside a rock, making it impossible to win the battle. However, I have encountered them in multiple locations outside that wall that are inaccessible to the attackers.
Apart from that, AI pathing for the gate to the inner castle seems to not work. Both attackers and defenders will run at the wall instead of through the gate.