Fixing the AI 20 Unit army stacks by turn 6

Is there any word on why this occurs? Or is there a mod that fixes it? I’m only six turns into a campaign as the High Elves when an army from a rival faction rolls in with a 20 Unit army and lays waste to everything. It’s infuriating. Have also had it occur on an Empire campaign and a dwarves campaign. What gives? Thanks for listening to the rant.