2 things:

1.  Had 2 armies , 4 units each, on 2 adjacent tiles.  Could not attack an opposing army with my 2 armies.  Had to engage only 1 army.  Meanwhile, although the other army "isn't engaged" and it is a multi-turn battle can't move it - zones of control? Both armies are 50/50 cavalry and immortals..

2.  The battle underway is stuck in limbo.  It is the other AI opponents turn to do whatever and nothing happening. So stuck in this turn, losing the battle as a whole army twiddling its thumbs while the other army gets hammered.  My idle second army's actions are locked while the targetted unit is executing pending orders.  opposing side not acting.  Maybe having 2 armies side by each has broken something.

I've encountered a similar problem. I had a scout who got challenged by an army of three militiamen. I retreated, and then the next turn I moved a separate army of three units up. The army of militia attacked me, I played out the battle, it extended to the next turn, and now the scout who originally retreated is trying to join the battle. It seems like since I have a spearman standing on the capture point, I can't add the scout or do anything. The battle is just stuck right now. Not sure if this is a bug or if the steps to move forward with the battle are just very unclear. Either way, it's problematic and super confusing.