For context, the AI has over 245 units on screen just during this skirmish, with others off the screen. They keep re-spawning over and over (despite their barracks and archery being destroyed). I sent in about 40 knights, and 60 other units and they got destroyed.
Restarting the mission creates the same issue. Prior, it would crash the game completely.
Look like the ai is using the game glich, to do free units, that are excluded from population limit, like some online people do, in multiplayer games. XD