This issue has happened with all of the builds thus far.
My friend and i enjoy playing games with both of us and 6 other AI’s in one match. We do 200 pop and all of the AI’s on the hardest difficulty. It doesn’t matter what map we are on, what civs we all are, or what the teams are set to but the AI stops playing the game around the time everyone upgrades to the 4th age. Vills stop reseeding farms, their troops group up on a random building and don’t move, and wood cutters stand still by the trees. We have tried to “poke” (attack a building or other item) the enemy AI’s to try and wake them up. It doesn’t always work but sometimes it gets the AI working again (by that time thought, the AI is so far behind that it is worthless). This issue makes large multiplayer games worthless.
Start up a multiplayer game with 7 other AI’s, the whole map explored (so you can watch the AI and what they are doing), set the population to 200, and play away and see when the AI stops doing anything.