See the below chart, yellow Mongol player’s timeline. It just stopped and started accumulating resources. Did not do anything, not even go to Castle Age.
First chart is vill count, second one is military count and the 3rd one is total resources.
UPDATE: After watching the repetition, the issue happens after the 1st ovoo depletes. The TC was next to said ovoo and the AI goes double vill. Apparently it won’t go anything at all if no ovoo is available. Worth to mention that the AI indeed made a new ovoo, but that did not trigger building migration, probably because the ovoo’s location is surrounded with the other player’s buildings (red player).