I encountered something similar in battle. It's related to the swamp.
By the way, this happened on a Chinese map.
I'm in the castle, I've stocked up on swamps. I'm standing in the corner, behind the "lepeshki" (presumably a type of defensive structure or terrain feature). And the computer AI isn't attacking!

It's convenient, of course, as I don't have to move. I just stand there and shoot. But... I don't understand why it's happening?
In other cases, the AI attacks, even if it loses a turn because of the "lepeshki," but it will attack on the next turn.