1.04 patch notes say:

-Fixed bug where joining a multiplayer game in a slot that was previously held by an AI player would have the AI still be enabled when you started the game.

However this bug is not fixed. We had it happen to us this weekend in a game. We were playing 4v4, 4 people vs. 4 AI, and one of the people dropped. We saved and continued, and shortly thereafter found another person to join the game. After sending the save file to that person, we hosted it up and continued the game. After the game started the AI was still controlling the dropped player's stuff.

We were able to work around the problem by going back to a previous AutoSave before the player had dropped, but nevertheless this bug is definitely not fixed.