In my current game 1.12 game with the Stormworld mod (on "normal' difficulty), I was steamrolling one AI's factions cities as quickly as I could without leaving militia or other units there to guard the newly conquered cities.
The AI sovereign and his stack left a city from the far west that I hadn't reached yet, and just stood outside one of its former undefended cities without reclaiming it for many turns (which it coud have easily done).
I don't know if that's a 1.12 issue or a Stormworld issue, but the AI should have taken the easy opportunity to get a city back.
Hopefully that will be addressed in 1.2
