I've had the passive bug a couple of times when sallying out, but not always. It's true that of what I've seen of this game so far this is the only thing that qualifies right away as a bug : it can be reproduced and the game cannot possibly have been designed this way.

As a wake up call for the AI, I have found that the enemy troops react as soon as I bring my general outside the walls, or close enough to them. After that, battle can proceed pretty much as normal.

I think it's also very easy to fix, a simple trigger on the AI, as opposite to the vastly complex dynamics of pathfinding and charging in formation. So I'm confident this one is going to be fixed in the patch.

I think none of the testers was sick enough to try what you did with catapults however.