The developers kinda botched mandate of heaven. They never fixed the issue of Korea going in to Manchu, and never fixed Manchu to not be a horde. Manchu being a horde and using horde mechanics leads to their ahistorical collapse.
Actually most AI can't use horde mechanics and they really should be looked at. Russia didn't get in to the steppes until the 1600s yet in game you can beat hordes in the 1400s...