Already reported in may 2013, june 2013, february 2014, etc: with Total War Rome 2, from time to time, the artificial intelligence player manages to get an army deprived of the reinforcements from other armies standing a few meters away. There is no explanation why it happens – the relevant armies are not using any specific stance, it does not seems it is the result of any spy work. Since multiplayer is turn-based (not to mention you cannot decide who would get a region after a siege held by two players), it is really a game killer,  since most sieges gives the artificial intelligence the opportunity to easily kick two armies with just one by taking them apart.