As I thought, devastated ruins are prone to fucking with the AI sometimes. Testing with the final book of Khorne and some AI such as Archaon is fixated on it. A few turns later, a third army of his is now there, all 3 just sitting there turn after turn. Often I'll see an AI sit at it for a turn maybe two then go elsewhere (guessing priorities such as nearby enemies approaching), but as Archaon has no nearby enemies, sitting at a ruin is top priority.

Bit of a shame, especially since I was hoping he'd get fat after all this time before I go and slaughter him.