Actually I was hoping to achieve a tactical victory here by first destroying the reinforcements and then replenishing my besieging troops and starving out that 1700 man army. Although that battle was a defeat, strategically it was a victory as I disposed of their reinforcement troops. But the game decided just to take one of my armies off the map. Second thing was my inability to bring all my reinforcements at once in my first try - a thing that the AI somehow manages to do.

Both things don't look like EB bugs, but it's very frustrating nevertheless.