It has been confirmed by several people over at com that the load siegebug was present in 1.1. Haven't read anything conclusive about 1.0 though.
So obviously something made the AI behave more conservative in 1.2, it could be that previously is was merely 'forced' to leave while now it forgets the purpose for its army's placement. Thus the army gets sent off to do other duties, which they didn't in 1.1.