[1.09p] Evaporating Sovereign

I was playing a new game today, and saw  a darkling and wolf pair approaching a city of mine which only had one defender.

I teleported my sovereign next to it, and walked in to give support. As I issued the order for her to move the combat initiated. I won handily with no losses, and at the end of combat I no longer had a sovereign.

I'm going to try to replicate it and save right before.