It is not really a bug, but I was attacked on a map with a Lake through a quarter of the battlefield.The main army was not affected, (Other than they tried to rout through through the lake and a few of them drowned.) Hoever, they had a reinforcment army which deployed on the far side of the lake outside of the battle area and some of them were in the lake and many drowned. Once the battle started, the survivors avoided the lake and wandered onto the field just fine.