The computer “doesn’t want” to play.

I have a similar situation: a hero with a strong army freezes in place, and it happens near treasure. I figured out the problem – he had the "Fly" spell. The AI behaves like this: if a hero initially has the "Fly" spell and an air mage, it will use it every turn, even if it's not necessary. And when the mana runs out, it will just stand there and accumulate it.
After a week of standing in one place, it flew again.