That does seem to be the case. However, I am almost certain that I had an attacking battle against two AI armies that did not result in a crash. It was as the Casse, against the Aedui.

I also have never seen the strange AI reinforcement behavior. I have fought quite a few battles with enemy reinforcements, and they generally maintain as good a formation as the original force.

I also have noticed increase occurrence of "delayed reinforcements," though it seemed to me like it happened most often in difficult terrain and/or winter, but I haven't tested it.