Have to say I've been playing 4 days now at Normal Difficulty level and I have'nt noticed this problem.

It's certainly a problem I came across with Gates of Troy but so far the AI in RTW has done quite a good job of organising its armies and attacks in my game.

The only real exception was an Egyptian attack on my colony in Phrygia where the AI sent is main army led by a Captain (stupid name for a non-character general) to beseige Sardis and a much smaller force under two of its heirs to attack Pergamum. On the face of it, it was a dumb move but it actually gave me a headache in that my garisson in Pergamum was blocked from moving to assist Sardis and I thought in fact that the AI was trying to be clever.

The same is true with fleets. Both the Greeks and the Egyptians met my ships with massed fleets with almost full stacks. However, I have seen these stack suddenly broken down into multiple small fleets in order to block sea routes or screen a vital troop convoy.

So, at the moment I am quite impressed with the AI and I'm a bit puzzled why the game should play differently for other people.